Don't overthink it. Our hate for Ann is based mainly on two reasons that have already been mentioned:
1- She's by BY FAR the ugliest model in the whole game. Her whole body has weird proportions, with a disproportionately big head in an underdeveloped childish body. She's been compared to a china doll, a bobblehead or a funko doll, all of which are accurate. For us, Emily, Alex, Jenny, Amber, the old dude's granddaughter, Ann's roommate, Vanessa, Vanessa's mom, the girl from the toilet scene, the girl from the library, even Mery if it wasn't for the venereal diseases, are all much more attractive than Ann. BY FAR.
2- Despite the fact that she's BY FAR the least attractive female in the game, she's always got way more sceen time than any other except Vanessa, who's had just a bit less than her. Our favourite girls being sidelined for years in favour of someone we find an abomination has aroused all that hatred. Simple as that.
It doesn't have to do with how ridiculously retarded she behaves in the corruption route. Retarded behaviours are all we see in this game when it comes to corruption, so if that was the reason we'd be hating every main LI. Well, every LI but Alex and Jenny and just because they haven't had the time yet to be corrupted. Give it time. Case in point: my favourite character, Emily, has fallen into the same absurd trend. The build-up with her feels ridiculous, as does that scene with Joe.