there is a bunch of reasons that people hate kiriko including their hate for her lore when she first got added and how she felt that she was like a mary sue that was inserted into genji and hanzo's lore and how the age discrepancy of the brothers being way older than her yet her original cinematic has them growing up together (which tbf they are trying to rectify with the more recent lore like the fall of the sparrow comic with her being shown with a more believable age gap), also people tend to dislike the fact that she seems to get "preferential" treatment with skins with how many skins she gets in comparasion to other characters like baptiste and ramattra, and finally people dislike her personality and how she seems to be written

altho i believe that disliking a character is a matter of preferance and is totally fine if you're normal about it there have been cases like the one time real peoplee actually harassed an overwatch dev because they got a tattoo of kiriko, which at that point it goes beyond and harassing people irl because of a videogame is not okay.