Female Equivalent: 膀爷 It's summer time guys the 膀爷s are out on full parade! Although I haven't seen that many females as exhibitionist as the males - there's still a fair share of ladies showing off their midriffs.

What's the female equivalent of a 膀爷?

膀娘?

膀婆?
Not sure there is one. 

膀爷 itself is more common used in the north. A quick search from google, neither 膀娘 nor 膀婆 reach more than a few thousand results, so that may be considered as an evidence that both terms are not widely used. After all, we can find 膀爷 (topless man) in many cities, but I don't think we can find many topless women in any civilized place.