Piranha Plant was just added to Smash ultimate a couple of days ago and Piranha Plant's gender "may have" been revealed to be female from Palutena's guidance.

[There should be a GIF or video here. Update the app now to see it.]

However, Palutena was actually talking about Viridi. But in the early commercials, the plant was singing in a girl's orchestra voice so perhaps the plant really is female after all.

[There should be a GIF or video here. Update the app now to see it.]

[There should be a GIF or video here. Update the app now to see it.]

"Sorry Pirahna Plant. I don't understand much Japanese... But that's definitely female."