I think the hate comes from an overall immaturity (I don't mean this as an insult) in the audience. Forrest Gump is a really solid family movie because its characters are thoughtful and offer many things for a viewer to latch onto. As a child, I remember not really caring for Jenny's behavior. It didn't make sense to me, I hadn't yet picked up on the sexual abuse from her father, her scenes were too sporadic for me to keep up with her character arc enough to care, etc. But I adored Forrest and loved watching him stumble his way into success. That was 20 years ago, and I've come to appreciate Jenny's character for how much depth she holds. When I rewatch the movie, it's because her story hits an emotional resonance with me. All that's changed for me is I've learned to care more about a life like hers.

I think it takes a certain amount of emotional intelligence to appreciate her character, and not everyone is there.