Because that it the name of the character she is costumed as.

https://en.m.wikipedia.org/wiki/Power_Girl

https://dc.fandom.com/wiki/Kara_Zor-L_(Earth-Two)

https://www.dc.com/blog/2023/06/01/everything-you-wanted-to-know-about-power-girl-but-were-afraid-to-ask