It's the swimsuit Poison Ivy skin.

Now, however, the ESRB has added “suggestive themes” to Fortnite’s rating, and that seems to be based on a single skin:

Players can encounter a female character depicted with deep inner cleavage and lower breast exposure.

This is clearly referring to the summer version of a Poison Ivy skin, a collaboration with DC that also featured swimsuit-wearing Harley Quinn, Catwoman and a swim trunks-wearing Bruce Wayne (mask on and off). And it’s true that Poison Ivy can probably be considered the most revealing skin in the game at the moment (though all of these are still miles below Marvel Rivals and certain none are R-rated)