I think most people hate Diane because they assume she’s supposed to be the good guy. I remember thinking “oh, so you’re just as flawed” in the earlier seasons and completely changing my perspective of her. but it’s likely most people don’t get that revelation. because, yes, she’s flawed. but when you think her role in the show is that of the perfect girl who calls Bojack out, she comes across as a holier than thou annoying asshole. which is, imo, the reason she’s so easy to hate. she was never meant to be the saint. but that flies over a lot of people’s heads.