I believe in this leak.
Everytime I see a leak, I can tell if there's something off or not, and this doesn't seem off at all, these mii costumes look totally on the same style as the rest of the ultimate stuff.
I mean, it could also be an amazing fake leak like the rayman one for smash 4, but I doubt that's the case.
I don't give a shit about Doomguy or Geno, this leak doesn't make me feel hyped or anything, which means I am not saying this because I want to believe on this leak and i'm blinded.
Take it as you want.