People's opinions on design remind me of my high school days when I used to scribble characters on my notebook to pass the time during classes. As an amateur with a little bit of skill, I'd just keep adding detail to make them cooler, so the characters would end up full of shit haha (sometimes the result was pretty cool though). I guess random people could think that I put a lot of thought/effort into those characters just because of how complicated they ended up being… but it's actually easy and fast to add detail, and I never tried to improve those characters after the first drawing (so not a lot of thought). They were just bursts of improvisation.
Unlike myself, Oda puts efforts and thought into all of his characters. You'll find even random ones who show up only for a page that have very intricate or unique designs, so the fact that Oda would not put an effort into a major characters is just insane, and the fact that this is presumed because of simplicity is just wrong. Imagine creating the main mink character of the series, who will be a major ally for an entire saga (at least), one of the top 20 characters in total chapters inclusions, but not putting a lot of thought into that mink design… Nah, that's not how things work, lol, unless the author had a stroke of inspiration and was finished in a few minutes, but very happy with it.
Oda probably had to draw Carrot many times until he got her right, changing hairstyle and eye shape, experimenting with sharper lines or rounder lines, trying between more animalistic features or a more human look, changing the clothes and having the idea for the weapon/gloves, and many other minor things that we take for granted but an illustrator has to ponder over, and all of this have to be considered in conjuction with what'll be the character personality, role and behavior. That's a lot of work that doesn't happen without a lot of thought... and it's not more effort to add a scar or a random detail, especially considering these kind of additions may be a bad idea if they don't reflect the ulterior vibe of the character.
Carrot's simplicity in design goes along with the fact that she's a young and inexperienced girl from a small country, and it's good technique to draw such characters with rounder lines and minimalistic features. And yet Carrot's face is unique and unmistakable even in close up, which shows good balance and judgement by Oda. Her short hair is puffy on the sides, which is different from any other hairstyle in the series that I can remember, and her battle gloves are unique. Above all else, Carrot is a bunny-girl, which sets her apart anybody else by nature, so the author doesn't need to go a long way to try to make her more distinguishable than she needs to be. It's totally okay to take advantage of the specific traits that are unique to the character (bunny-ears, tail and nose) in order to have them be the differential.
In comparison, it's much more important for Yamato to have more details since her template is Nami's face, so there's a little bit more of a struggle there. You need to add stuff to hide the similarities, which is a good and a bad thing. For what it's worth, I like Yamato's design a lot, but there's this "uncanny valley" feeling that I have any time I look at him, so it pisses me off a little bit that Oda couldn't have given a different face to Yamato. But I like all that's new there. I just want to see him in a different outfit to make sure that Yamato is different enough from Nami.
Another important thing about character design is the sillhouette, like being recognizable only through your shape, like that game "who is that pokemon?" from the anime back in the day. Carrot has a distinguishable sillhouette, not only for obvious reasons (bunny ears and tail), but because of her hands (the gloves make them big) and because of her many action-cute poses. Yamato also passes the test because of his club and the clothes.
All in all, it's essential to have your main characters looking distinct and memorable, but this is not done simply by adding more detail. It all depends, and a lot of it comes down to the character being recognizable. Sometimes distinction happens strictly because of the author's art style, and then any of your characters are unique because of that. You'll find many MCs in manga/anime who look basically the same, but the art style of the authors is different as well as the outfit, so it works. Oda gave Luffy the straw hat and the eye scar, so he looks more unique than usual for that type of boy protagonist. On the other hand, Nami is extremely simple and the main girl of the series. So you can see that Oda works in both spectrums. What seems to be the rule is that Oda opts for relatable main characters with charming designs for a teenager audience.
As far as I can tell, both Carrot and Yamato have designs good enough for next Strawhat.