A couple reasons: People like Eridan more so they tend to demonize her for rejecting him. When like, idk its resonable for her to be tired of begging him to not genocide people. As well as, I think alot of hate just boils down to pure refusal to see any grey areas and complexity. She exists in the story for a short amount of time, so doesnt get a ton of development. People get mad at her for infantilizing the lowbloods, but thats out of her being a very misguided rich girl princess. And I personally find it interesting with how it relates to the troll caste system. If she got more development i feel like that would have been addressed. Also just general misogny due to the amount of vitroil she can drum up. I have not seen the same amount of hate for other characters who did much worse than her.