Its no secret that Sakura is probably the most disliked character in the Naruto fan population. The most common reason Ive generally seen is that shes "useless" but I dont think thats true, her overall feats throughout the manga far outweigh any other character in the K11 bar Naruto.

I think the reason she is hated is simply her personality/character: shes whiny, temperamental, loud, impulsive and sentimental, i.e., she is the embodiment of the word b!tch (its like shes on her period 24/7 and this is especially true of the animes portrayal of her character).

When I watched anime, I developed an instinctive dislike of her character for these reasons, I see complaints about her role in the manga as rationalization tbh, I think those of us who dislike her simply dislike her essential character traits.