Jenny was sexually abused as a child and believed she was sexually abusing Forrest. This is why at the end Forrest tells her "I may not be a smart man, but I know what love is." He's telling her that their relationship was consensual and that she was not taking advantage of him.
It's fine if you still believe she's an asshole to Forrest, but most folks don't understand this context.
It feels so weird to see her character compared to cartoonishly evil ones like Darth Vader and the Joker. When watching the movie she is clearly a very damaged person, her character is tragic. The way she treats Gump makes sense, her culture is intensely ableist which combined with her trauma provides context to her actions. The idea that she is somehow a bigger asshole than a domestic terrorist with no concrete backstory, a serial killer, or even evil space jesus, is ridiculous. Frankly it reeks of casual sexism whenever I see people smear her, it feels like folks are either parrotting the opinions they found online, or never tried to understand her character while watching the movie.
Listen, we all know Darth Vader may have used the threat of genocide to get Leia to talk... And he may have even lied and done it anyways... And just the general idea of genocide and destroying a planet is awful... But have you considered women bad?
We should also acknowledge that, not only did she wait until she was dying to tell Forrest about their son, there's also every possibility in the world that she was lying about him being the father in the first place.
As far as we are shown, they only had sex one time. Meanwhile she had recently arrived afteryears of a destructive and careless lifestyle including sexual promiscuity. And after they had sex, she left the next morning. It's not impossible, but the odds that her pregnancy resulted from her one night with Forrest, rather than her sexual partners before (or after) she showed up at the plantation, are pretty low. Especially given Forrest Jr. did not inherit any of Forrest's mental handicaps (an is in fact of higher than average intelligence).
The odds that she was dying, didn't have any clue who the actual bio-dad was, and so lied to and used her handicapped, lonely, yet wealthy childhood friend as a means to secure her child's future regardless of his paternity, though, is way higher. None of that is confirmed, obviously, but... it doesn't look great.
Especially given Forrest Jr. Did not inherit any of Forrest's mental handicaps
Did they ever specify any conditions he had? I see zero evidence that it was hereditary. Just because something "doesn't look great" doesn't mean it is true. It's not supposed to look good. It's supposed to look bad. The film is not trying to absolve Jenny in that moment.
Seemed to be a general learning disability. A parent with a learning disability is more likely to have a child with a learning disability. It's not necessarily hereditary, and I'm not saying someone with a learning disability can't have a gifted child. But given the whole set of circumstances, it just compounds the likelihood that Forrest is not Forrest Jr.'s biological father. It's not impossible that he is, but it's far from a certainty.