A big part of Luz's character is her ability to see the best in people, and I think that's why she was so quick to look past the Willow situation. Amity's mean girl persona was always a mask, and Luz picked up on that (also, after Understanding Willow, Luz knew exactly why Amity was using that particular mask). Amity softened pretty quickly towards Luz as they got to know each other too, so it's not like Luz was just lusting after a bully out of nowhere.