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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