While convincing, more so than plenty other leaks out there, I still think it should be taken with a grain of salt, like with all leaks. From what I've heard, Mii Costume mods aren't at all that common/easy to make on the modding scene, but I think it's entirely possible someone could've put in the hard work to make hats (both costumes are hats instead of full costumes) just to make a convincing fake and shake up the speculation scene.
And if these costumes
are real, Mii Costumes don't 100% confirm characters, as of late especially.
Here are the current waves of Mii Costumes that released along with new characters, current as of Challenger Pack 3. Waves 1 and 2
did have costumes that corresponded to their character, with Wave 1 tossing in a couple Sonic costumes from 4, but Wave 3 consists only of new or returning costumes that have nothing to do with Banjo. And given the cameos on Spiral Mountain and the pool of spirits, I don't think Nintendo had trouble with getting Microsoft to let them make Banjo series costumes if they wanted. Goemon doesn't confirm a Goemon series rep, nor does Sans confirm an Undertale series rep. The Mallow Hat and Cacodemon Hat could just be similar gestures from Square and Bethesda as the Sans costume was from Toby; a form of acknowledgement of the fans' demands for a certain character to be playable, even if it doesn't fully pan out this game.
And even if I've missed the mark entirely and these are real and/or Geno and/or Doomguy gets in *deep inhale*... I'd still say Wright has a chance, and I don't think Geno/Doomguy getting in discredits him further. He just might be further along on the schedule, we can't really say for sure yet until we get a final, definitive say that so-and-so is the last DLC character for Ultimate, and Wright is still one of the strongest third party contenders for a slot in my opinion. I'm also not hoping this isn't real or anything out of spite or hoping the character I want gets in over someone else's, just giving my two cents on the leak since discussion about it's come up here.