Powergirl should be deliberately played as a contrast (but not outright opposite) to Supergirl. They really are aimed at different demographics. Supergirl was made in the 50’s and has always tended towards a more idealistic “wholesome,” aesthetic aimed at young girls. Powergirl was made in the 70’s a time when superhero comics were starting to skew older. Powergirl is edgier then Supergirl. Her personality is more abrasive, shes more impulsive, and yes her costume is more provocative. She really seems aimed more at teenage aged boys. It’s like her creators said “you know all those ideas that are too extreme for Supergirl. Let’s spin them off into their own character.”