@Chams:
We all had concerns over Brook joining so suddenly, but he became one of the favorite members of the crew. I don't like Carrot so far, but if she ends up joining I'm sure we'll end up liking her as well. So, no worries.
There wasn't much sudden about him, since he had everything going for him in being a Straw Hat. He had a goofy, unique personality, and was the musician that Luffy had always wanted for the crew. His intro arc was "his" arc, where the entire plot for the arc was carried by what he needed. It's just like how Luffy helped Zoro escape from execution, how the crew helped Usopp with Kuro, how the crew helped Sanji protect the Baratie, how the crew helped Nami with the Arlong Pirates, how (some) of the crew helped Chopper with Wapol and his gang, how the crew helped (honorable crewmate) Vivi with Baroque Works, and how the crew helped Robin with CP9 (when she actually considered herself a real member of the crew). Jinbe has really been the one exception to this in the buildup to his joining, but he was a big part in helping Luffy through Impel Down, through Marineford, and through his depression. Then Luffy "repaid" the favor by helping out with Hody and his crew, in order to further Fishman-Human relations.
There have been dozens of candidates (side character or not) that people have latched onto at one time or another. During Dressrosa, there were countless posts in this very thread about some of the eventual grand fleet members being Straw Hat candidates, because the existence of a Grand Fleet just wasn't there yet (but it was obvious and used as a counter argument to them). None of them were Monet or Rebecca level characters at all, but rather were side characters that Oda gave focus to for a good portion of a very long arc. It's very similar to the buildup of Carrot, where her likely position after all of this will be in a Mink Faction in the Grand Fleet.
Carrot really hasn't gotten anywhere close to the amount of development as the current Straw Hats have. She's just been depicted as one of the side characters along for the ride. She's certainly gotten more development than the likes of Monet and Rebecca ("impossible candidates"), but so have characters like Kin'emon and Momonosuke. They've both been with the crew since 2012, and will likely be with the crew until well into 2020 at th
And on that note, the whole lookout position is a very weird overall argument for a character joining, since it has never been seen in a prominent pirate crew as a named position, and hasn't had a mention at all by the Straw Hats. A helmsman was always something we were going to get (even if it wasn't going to be their reason for joining). The Blackbeard Pirates had one, and the Straw Hats are the mirror of that crew. Someone is bound to say that an archaeologist was the same way, but it a necessity when we were introduced to the existence of Poneglyphs. Reading the Poneglyphs has really been the most important use of that role for a long time now, and we have characters like Oden on the Roger Pirates now as equals to that.
@AvocadoInTheRain:
This website is really the only place where people generally don't think she'll join or don't like her. On reddit, tumblr, and 4chan the consensus is that she's wonderful and that its somewhat likely she'll join. Fan reaction is pretty positive towards Carrot. She's the 27th most popular character of all time on the poll, and that was made before all of the stuff she did since the wedding. Sulong alone probably bumped her up a dozen spots.
This is how the general fandom has always been, though. People have always latched onto the newest character that gets any bit of focus beyond being a background character, and it moves on from there until the Straw Hats say goodbye to them. There's always posts of "I'm 100% sure this character will join, and here's why". I've read threads from each of those websites, too, and there's always people chiming in and naysaying each of the potential crewmate discussions, and that definitely includes Carrot too. There is never a consensus.
There's also been countless theories on Jinbe "dying" before he can join the crew, on those websites, and even here. Random theories are always going to be a thing in a fandom of this size.
And yeah, popularity polls don't mean anything at all.
@AvocadoInTheRain:
Some people though, have been saying they don't like her personality as though that were a reason why she won't join (coughBelow Averagecough).
I don't think Below Average's point is that her personality is bad as a "character". It's just bad for a "Straw Hat character".