Personally, I think her storyline in the second movie really suffered because of her age change.
By that time, Moretz was also the type of young woman who was taking big roles in projects like the Carrie remake and If I Stay and didn’t really need to be propelled forward by the Kick-Ass franchise anymore.
Chloë Grace Moretz aged three years between the original movie and its sequel, and by the time the second film was coming together she was no longer an adolescent but a young woman.
Hitgirl in particular’s storyline is rough in Kick-Ass 2.