If something or someone exists, there’s no reason you can’t be on Fortnite. In terms of probability, I would rate Will Smith's new appearance on Fortnite at the "probable" end of the scale, because it's not as rare as, say, Martin Luther King's "I Have a Dream" Speech broadcasting in the whimsical promotional vehicle-slash-video game about the murder of other players. In addition: a data analyzer unearthed the skin last month.
It is not really a skin of Will Smith by the way: it’s a skin of the Bad Boys character of actor Mike Lowery. It’s available at the Fortnite store and includes Detective’s Duffle Back Bling and a double-thrust pickaxe called Loose Cannon Cutters. I'm sure there are many reasons why it's called that.
Why Will Smith (or Mike Lowrey) at Fortnite all of a sudden? Is there a reason? As far as I know, there aren't any. Season 7, Chapter 2 is based on the aliens, and as several sites have pointed out, Mike Lowrey doesn’t fight the aliens, he never did, although there are many other characters in the Smith movie.
Anyway, you can now be Will Smith at Fortnite and that’s something. Lately, skins for Morty, Wonderful woman i Ariana Grande have been added to the game.