Make her a loving but bullying big sister type.
Someone that gives her teammates or just the people around her a lot of shit but it's to nudge them into doing better because she really believes in them.
America herself would be an over achiever. Teleporting around to different charities when she wasn't doing super hero stuff. Always busy always on the go.
Overtime or eventually you reveal the reason she's acting like this is because she's scared of being alone or has an aversion to dealing with solitude. So she's been pushing people away and keeping busy to avoid how out of place she feels. Make the reasons being separated from her parents and the Young Avengers going their separate ways. She feels abandoned but sees herself as too strong to admit or bother others with it.