Thank you!
Mii Fighters costumes have two problems: 1) as far as I know, Mii's costumes haven't official artwork; 2) Miis are personal and to see another one's face could be odd. But I'll search the renders (or if you find them let me know) and I'll try
For the premium Miis I listed, that part really wouldn't be a problem as the headgear is a full head model that makes the Mii look like the character they're a costume of.
The first issue is definitely something I had thought of as well, but I don't think they need to be Smash specific renders, just some great 3D renders out there with a transparent background that could work.
Here's some examples that would probably work well I think:
Hopefully those shadows wouldn't be too problematic, especially for Cuphead. Finding a decent 3D Vault Boy render with a transparent background is proving to be a bit more difficult to track down than the others though. I'll post and update if I find one though. I'm also not crazy about the Bomberman render, as it's not based on the Mii and so his legs look longer, and he has the angry eyebrows which he doesn't as a fighter. So I'm trying to find a better one of those too.
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Here's the best Vault Boy render I could find
But he definitely suffers the worst from the shadow around him, I imagine that could be problematic to work with. And here's a Bomberman render from his Mii costume coming from the same source, yielding larger shadows as well:
Don't love Bomberman's pose here, but at least this one is the actual Mii costume.