Charlie is quietly insecure which is why I think she acts the way she does, she has been fixated on Joe for a longtime - always popping up in the background of their relationship. As of this, she uses the opportunity of the break up as one to pounce upon Joe - something that infuriates Alex, as they were all almost friends at a point. She will do anything possible to get Joe to like her, sometimes by imitating Alex and saying bad things about her. Her insecurities lead her to portray a false confidence, so her attitude appears cocky at face value.