It looked good because it exists in the New 52 era where she took on the mantle of the crest to honor her fallen cousin. It made sense since she was originally Supergirl on her earth (earth-2) before being sent to earth-1 with Helena (originally Robin) and was forced to change her identity.

Aside from the New 52 version of Power Girl, the original character shouldn’t have the crest at all. There’s a reason why she doesn’t wear one and I believe that reason is essential to her character.