I wouldn't say she's hated by most Blinks. I'd say she's hated by many Blinks. Blackpink in general has a lot of solo stans who can be toxic by talking badly about the other members. With that said, I'm sure most Blinks support all four members of the group.
To answer your question more, hate towards Jennie has a long and complicated history. Many Blinks and just K-pop fans in general felt from the very beginning that she was the most favored by YG Entertainment. They got the impression that she's too much of a diva who thinks she's better than everyone else, which I think is untrue. Even if it's true that she's favored the most by YGE, that's not her fault.