Not liking Peeta because he has a stalker crush on Katniss and watches her walk home every day for ten years is fine. Not liking him because he’s “a wuss” is actually just misogyny, and it means you don’t really hate Peeta, just traits that are traditionally associated with women.
Let’s do another one. Hating Peeta because he probably could have managed to help the (obviously starving to death) girl of his dreams a little more than he actually did is fair. Hating Peeta because he didn’t kill any children in the Hunger Games is not cool. Hating Peeta because his motives with regard to Katniss often seemed sinister is ok. Hating him because he gets hurt a lot and needs Katniss to help him doesn’t even make sense.
One more. Hating Peeta because you think Katniss is more compatible with Gale is fine, and would probably make for an interesting discussion. Hating Peeta because you think Gale is “more of a man” or he “deserves” Katniss in some way is problematic at best.
A central theme to the books and the movies is Katniss learning things about herself and discovering who she loves and why. Her feelings toward both of her love interests are constantly shifting. In addition to presenting us with several different types of women, we are shown very complicated pictures of masculinity in this story, something many films entirely full of men don’t usually manage. If you hate Peeta because he’s “not manly,” then you are being presented with an opportunity to think about what being “manly” really means.