Grima Robin's art?

It's probably because of how different it is. Compared to other art in FEH, you either get the "too anime" like Mist, Rebecca, Lachesis, etc... or you get the "is this outdated..?" like Eliwood, Gunter, etc.

The "comic" ones are like Arthur, Grima!Robin, Raven, etc.

edit: whoops i rambled on...

tl;dr:
Robin's art is very different from his other depictions because he is not really Robin while Celica's depictions isn't very different because she is still Celica. This is a thing IS does to differentiate alternate versions of characters and their different personalities somewhat (i.e. Tharja v. Winter Tharja and Lucina v. Spring Lucina)

yeah i get what you're saying...

it seems like they're trying to go for an edgy appearance and Robin's might just be too much that it doesn't look as appealing and just looks kinda lanky. His attack/special art tho, is so much better than his base art. more dynamic and the expression on his special art is just stunning.

Okuma Yugo's strengths is probably not in neutral poses which is why it looks kinda odd, like its trying to move but its kinda stiff. But it still feels like they did go all out and did too much for robin. probably the same reason a lot of ppl don't like Ogma, Jagen, and Arthur's art. It seems out of place. Aside from Arthur, Ogma and Jagen's art has way too much detail and they stick out too much in a game full of very cartoonish characters. From Yamada Akihiro's work, people dont have much of a problem with Zephiel or Gunter's art because the details are kinda watered and toned down. Still recognizable as theirs, but does not stand out as much.

So like compare RobinM, W.Robin, and E.Robin. RobinM and Winter Robin have a simplistic cartoonish artstyle going on, then you throw in another one of him and suddenly he has a whole different personality. We have three versions of Azura, all drawn by kaya8 and we can understand shes Azura and she doesn't act any different from the Azura we know. Then we have Tharja, drawn by ZIS, who's voicelines reflect her character and art which is followed by Winter Tharja who is drawn by Washimoto instead. Washimoto depicts her as "Cutesy" compared to her original version and her voicelines reflect this somewhat, he actions still being the same kinda but ultimately different in tone. Tharja is pretending to be Normal and the difference in personality is reflected in her art. This reflects Celica being drawn by Fujikawa Akira again. She's still fighting the possession in her voice line so we can assume that is still Celica in there.

I really like that art style, it’s very different. At least he doesn’t suffer from ‘same face’.

I actually feel like Grima Robin looks kinda wrong. Not like possessed wrong but somehow...not human. Probably not too bad for this case but I don't really like it. At least not the normal artwork. If I get my hands on him I'll fodder him off instantly.

I dunno, I really love it. It's pretty wacky, and it's overly-detailed chaotic appearance reinforces that Grima Robin isn't quite as human, and how he's somewhat a monster. The special art though.... I drool. Imma bout to run Moonbow on that boy just so I can see it ever other turn.

I agree, there's definitely something off about his art. It doesn't really fit at all. I would have loved to see a more realistic version of it, instead of one that looks like it was taken from some unofficial comic.

It looks too much like fanart imo, really unprofessional.