Not sure I really hate anyone and typically I end up liking everyone more as times passes. But if I'd have to pick out some to be ones that I got some sort of beef with...
Merry - I just hated how some streamer got their followers to sway a popularity poll in her favor which resulted in her suddenly taking a popularity upswing and become such a top tier dreamie for so many people. It just all felt so fake. Also her double chin making her fat just makes me think of Bart Baker's "All About The Bass" or "Me Too" parody with the fake double chin. Merry is a Youtube parody man cosplayed as Megan Trainer is what I get from her when I look at her face. I'd rather just get an actual fat cat.
Sherb- Don't want to say it too loud cause he is on my island, but I find him overall overated. I use to generally dislike him for it, but now I have him on my island and can't help but love him some. He asked to come over to my house today and stayed for AWHILE, so I mean I get he has appeal... just not nearly as much as everyone else seems obsessed with him. He just makes me think of Jimmy from Ed Edd and Eddy as he just looks slight sickly. He dodged the move out bubble yesterday as it struck Sheldon, but I still see him leaving... more than likely.
Dom - Never understood the hype on this sheep, he's so basic. Cyd, Reneigh, and Megan are far more interesting in terms of design and are drastically overlooked for this plain sheep. I don't hate him and wouldn't mind him on my island. I just don't see how he exploded in popularity. Audie, Judy, and Raymond made sense to me. Dom did not.
Rocket - My starter Big Sister that refused to move for months. She was weird and aggressive. I think the Big Sister dialogue really has to come from a villager you like or it'll rub you the wrong way since they have attitude. Since I didn't like Rocket and she stayed awhile... I don't really care for her. She is still part of my AC history though as the only true disliked resident of my island.
Marcie - I hate her design, I find her repulsive and she has not grown on me at all. I still find her hideous. Simply giving her a nose would help her out. But she is overall a massive mess to me. And her baby is all dolled up which is creepy.
No noses, Bad fringe haircuts or bangs, Rabbits with mismatched colors between their heads and their arms/legs, Black soulless eyes, Forever shut eyes - These are just design aspects I generally dislike. Typically this feature doesn't sink their entire ship for me, but it is something I hold against their designs!