@Count:
These are my thoughts on the impressions I got from Pekoms, Tamago, and Katakuri:
! Pekoms and Tamago had panel time before this arc, but that panel time wasn't anything notable for them specifically. Tamago and Pekoms were only Big Mom's delivery boys and didn't really stand out as potential antagonists, or characters that were really strong. The only noteworthy thing they did was when Pekoms punched Caribou, but the Monster Trio could easily do that same thing. His bounty was also lower than Luffy's. They themselves didn't carry so much hype as much as gathering hype around the crew that they belonged to and hinting as to what themes their designs, powers, and island(s) could be centered around. Bobbin had a shady introduction scene of coming back from destroying a country and having random henchmen, but all he did was deliver some news to Big Mom. I do think Bobbin playing so little of a role is disappointing since that country-destroying statement and not being afraid of Big Mom while Pekoms and Tamago were could have gone somewhere, but it wasn't a big loss at all.
! I am only disappointed about Pekoms' lack of relevance in Whole Cake Island purely due to his connection to the Minks, Bege, and Straw Hats being sidelined. But we've seen this type of thing before with the likes of Conis and Margaret being early arc allies that then got mostly ignored when bigger characters appeared. And I am disappointed about Baron Tamago not being relevant outside of his fight with Pedro only because he easily ranks in the top five or even top three of the best designs in Big Mom's crew. There is so much quirky creativity in his design. He's French Mafia Dr. Eggman with a cane. I would easily swap out his design/personality with one of the Sweet Commanders if I were Oda. Probably Smoothie if we're being real here. But outside of that, there's little to actually be disappointed about him plot buildup-wise. He was picked for a tribute retrieval job, he did it, and then said Bege wasn't at fault for shooting Pekoms a few arcs later. That's fine. Unlike, say, Diesel getting taken out in one bite from Carrot without even throwing an attack in a plan formed by Chopper off-panel. After his silhouetted intro, that was underwhelming. At least Randolph attacked the Straw Hats in the Seducing Woods one or two times before Cracker upstaged him.
! But yes, Katakuri is a bland character that is missing something, and comparing him to the empty characterization of the Vinsmokes is quite apt. I appreciate designs based on a mix of how they uniquely stand out silhouette-wise, how stylish they are, and above all else how they express the motifs of that character. Or in short, semiotics. When I look at Katakuri, all I think of is Oda taking Guts from Berserk and shoving random stereotypical badass attire into the design. His design says nothing besides "cool badass" in no particular authentic style, and his characterization unfortunately boils down to pretty much that with a seasoning of focused effectiveness. I do like how he stands out from how goofy all of the other Big Mom Pirates look, but I also worried that he looked too different from them to the point that he could fit into any other antagonist crew (which is why I appreciate the latest anime opening revealing him to have hot pink/red hair, which I hope that Oda sticks with official manga colors). Being serious, stoic, and competent are not enough to make a character memorably interesting either. They are not even real personality traits in my eyes because those descriptions could be executed in a whole bunch of different ways (you don't need to be scowling all of the time to be a competent villain that gets things done, and you can have other emotions and depth to complement not being as emotive as other characters, as well as unique manners of speaking and acting). At best, a character like Katakuri only works as an effective foil to other characters in showing what they aren't like, which is sort of the only main reason why his design and personality ever get praised. Which is somewhat sad since you're basically saying this character's only merits are in standing out in one certain way compared to other lackluster characters. The first time he ever showed any real character was when he mentioned wanting to protect his younger siblings.
! Aside from the scarf and novelty of one of the Big Mom Pirates' unexpectedly looking like such a hardcore punk, I only like his powers. And only when his powers were revealed to spawn multiple limbs that act as more powerful freakshow versions of Luffy's attacks. Outside of that, he's boring to me. But he can be very effective in giving Luffy the training he needs to prepare for Kaido in the next arc.
In regards to the topic at hand about Katakuri being an underwhelming for a major arc fight opponent in and of itself, I'm fine with him in that area. He only stands out because Oda has never deviated from shifting who the main antagonist of an arc was aside from maybe Marineford and Kuma confronting the Straw Hats at the end of Thriller Bark. The final threat of an arc does not have to also be the main recurring threat of an whole arc or be hyped up in a previous arc. Not that they should come out of nowhere as a random entity at the last minute either (coughNARUTOcough), but he debuted exactly twenty chapters ago and was easily the most recurrently standout character in the wedding and chase aside from Big Mom, even compared to Pudding. Perospero is only just recently standing out on a similar level. Katakuri earned enough of an impression to feel fitting and satisfying for a climactic one-on-one fight with Luffy in this arc's final act, especially since we have become familiarized with how imposing the Sweet Generals are ever since Luffy fighting Cracker in the first third of the arc and needing Nami's help to defeat him.
Luffy's fight with Katakuri is not on par with the emotional/thematic catharsis of past final arc fights, of course not. But… we don't need that in every single arc. It's fine to switch things up here and there, especially since it makes those kinds of fights more special rather than feeling so rinse-and-repeat. Katakuri does not need to be a memorable villain so much as a memorable threat. You would only have a real point if we were going to see the Big Mom Pirates defeated in this arc, but we aren't. It has been a survival/escape mission, and most of the Big Mom Pirates are forgettably one/two-dimensional. All Katakuri needs to do is deliver on some good suspense as a powerful opponent and push Luffy to the limits necessary for improvement in the next arc.
With all of that said though, Katakuri is not going to be the final fight in this arc. It's going to be Big Mom. Not in the standard way of overpowering her, but feeding the cake to her in a fight that might be styled like the team-up against Oars. There is no way Oda is letting Sanji save the day with the cake without having to share the spotlight, Luffy is going to at least hog half of that when he breaks out of the Mirror World. I fully expect this fight to end in a stalemate or an interruption after Luffy shows some sort of battle progress in analyzing Katakuri's fighting style. Ending an arc so centric around Big Mom and the kingdom getting destroyed with Luffy fighting a Sweet General in a remote area away from the destruction or any prominent witnesses besides Brûlée and random chess soldier isn't exactly too remarkable in the scope of this arc. It's going to focus on the big bad Yonko getting calmed down by the future Pirate King. If Katakuri showed up on Zou instead of Jack and fought Luffy there instead, that would be fine since Zou is a shorter arc with much less stakes, conflict, and characterization focus than Whole Cake Island.
And this is me random rambling a bit, but she BETTER not become unconscious after eating the cake while her family lets the Straw Hats go as thanks. I hate it whenever people suggest that as a way for this arc to end because I feel that is disrespectful to the amount of focus and characterization Big Mom has gotten as an antagonist. I get that she has a belligerent-incuding illness and the character flaw of scaring her children into becoming hazardous yes men, but she is not supposedly incompetent and pathetic to the point that her children need to make the decision of letting the Straw Hats go while she's peacefully sleeping. Luffy and Big Mom declared this "war" between each other in Fishman Island, and by God they should be the ones to end it as captains on an equal level of respect.
Rebecca was one of my favorite parts of Dressroba, so my taste might not be the most trustworthy, but i think Dogtoogh has been my favorite character post TS, above even Jack. I consider Dog to be one of the most well fleshed out characters in series, and this is done thanks to Odas improved writing skills, or at least what he has learned after 800 chapters. When i say fleshed out, i mean the potential to be an actual human being, and in that way Dog shines, and its really something else considering how little we've seen of him. What do we know about him? He is the second strongest big mom pirate, and in the world there are few who could beat him, he's also mastered his CoO to a ridiculous level and has a devil fruit that is in a way maybe better than Luffys. Oh and his lack of "quirks", or "speech pattern" or whatever else. I know this is a manga that flaunts how over the top the characters are, but in his own way Dog is also over the top. Just not in the way we're accustomed to. He has a personality, an already very defined one, but in Odas usual manner his look is just over the top, but in this case with an archetype we are not used to seeing from Oda. As you put it, the badass, well guess what? That in and of itself is more unique in this manga than anything else, and the lack of "quirks" is to make his personality and character work. What would be the point in having him do stupid things in serious situations just like everyone else? What exactly does that even add? Nothing. And why do i say that he is fleshed out and has a very believable(one of the most believable and sound characters in the series) personality? Because of what he's done and said. In almost every scene hes either trying to protect his family or being completely realistic and to the point, a trait Oda obviously put thought into, as someone who can see the future to some degree must have that ability affect his manner of acting and thinking. By realistic i mean what he says and does is mostly common sense(unless he is prevented from doing something by Oda himsel, like kill Luffy), examples like saying the main island was done for when BM started the rampage and him waiting for Luffy.
Also, his way of acting and what hes said and done are what make him a standout character to be honest. Let's be real, BM is a god damn mess. Shes' nothing but a wreck that happens to hold a massive amount of power. But in spite of all of that shes still Dogs mom, and all those freaks are his siblings, and what have the SHs been doing? Destroying his home and beating on his family. He is hardly a villain and to be honest hes', in terms of morality, actually a good guy compared to Luffy and his crew at this point. Him, as the second son, as someone able to see the future, is extremely protective and since all hell is breaking loose it is ridiculous to expect the guy to joke around or have a "quirk". For this guy, who cares so damn much this is serious business, since he has to literally watch over the kingdom AND his own captain/mom.
My hat is off to Oda, Dog is an incredibly well written character, and revived one of the least interesting arcs in the series. That and big mom being fun in her crazy mode.