Not so spoil anything too major but when her opinion of MC does change it doesn't come about via a shallow idea of "well he's hot now" even if she does find him attractive. If that's all that changed her mind then yeah, that's just terrible writing and I wouldn't expect anyone to sympathize with her or like her if that's all that changed with her. I think a good comparison for Tiffany would be a character like Catherine from cosy cafe (if you've ever played it). Someone whose meanness comes from their circumstances and is more surface level than perhaps a genuine reflection of who they are, who only needs to be given a chance to be better and then they will be better
I'm saying this more because before he couldn't defend himself, and now he can: If the MC still couldn't defend himself, she would continue bullying him. And she wouldn't have done anything to stop her boyfriend (who seems to be the ringleader) from continuing to do it too. No matter how hard you try, I don't think I'm going to like her.
I'm not going to play Cosy Cafe because I started it and didn't like it, so I don't know who Catherine is.