I think that unlike Ishida's childhood "friends" and classmates, Ueno genuinely liked him and had a crush on him (that still continues to today). So when the Nishimaya bullying finally blew up in their (Ishida's) faces, she wasn't able to just forget about it and turn the bullying to Ishida. Obviously she wasn't brave enough to face the social risks and help Ishida, and instead she turns the blame to Nishimaya for "causing" all of this (though I think she admits in this last chapter that it was kind of their fault too for not understanding Nishimaya). By seeing the guy she liked tormented so much she was hurt by the situation as well, but has done an extremely poor job of dealing with it. Rather than trying to change herself like Ishida she's simply taking her anger out on Nishimaya in some vain hope that Ishida will go back to his old middle-school self..
Hopefully that comes off as somewhat clear. Basically I think the Nishimaya "incident" hurt her as well but she wasn't able to deal with it appropriately and is now suffering the consequences. If her reaction in the Ferris Wheel was any indication, I have to imagine her life isn't going too well either (I might be totally imagining things but I'm thinking back to how she was so timid and hidden when Ishida first ran into her again).
I might be thinking into this way more than I should but I find Ueno to be an extremely interesting character, much more than most characters in the manga I read haha