Hey, know what you did there? You cherry picked costumes of characters whose face is revealed, and also Chibi Robo, which has a relatively easy explanation. Of course, it just so happens that most Mii costumes are of human characters with just their iconic hairstyle or headpiece being used to be completed with either your approximation of the Mii's face, or just your custom Mii dressed up. That's the point to them.
See this? This is actually
really good. People sleep on this because 98% of Smash fans probably have no idea who he is.
If Miis could be customized just the
tiniest bit more, he would be perfect. Why do you think that is...
Oh, right, because Goemon has always been stylized as a stocky little bobble head man. Go look him up yourself and see.
My point here is that Mii costumes are typically unfortunately of fan favorite characters that come from a very anime-esque franchise. Zero is a five and a half foot tall robot with very inhuman proportions (lower leg longer than thighs - a common trait among Mega Man and Mega Man X characters), as are Isaac and Rex. They don't "work" according to how a fan would perceive them as ideally playable. If Miis were taller and lankier, they would work better for anime style characters and worse for cartoony characters - in such a universe, people would probably hate Sans, just by the way.
Just kidding, his fans would still love it because like I mentioned already, even they didn't expect anything.
"So", I hear you asking yourself, "why does Chibi Robo look like one of those guys dressed up spinning the arrow to point you to the title loan office?" and I did say it was easily explained - look at Chibi Robo's body and look at the Mii. Just look at it from a completely objective standpoint, don't be offended for Chibi Robo fans and don't be angry if you're a fan yourself, just look at it. See how his body is disproportionately massive compared to his head, and how his weird spindly legs and arms had to be converted to fit the Mii fighter? This is the same reason we don't have echo fighters of characters who are taller or fatter than their base form - a true to form Chibi Robo Mii costume would have had to either adjust the hurtbox of the Mii fighter, or kept the hurtbox while changing the player model, which would have made it the optimal costume for mixing up your opponent's perception of where to hit you, and likewise, would have made it the optimal costume for not knowing where your own fighter can be hit.
"Ok", I hear you tentatively agreeing, "but why isn't Sans just his head on top of a Mii's head? It happens to a lot of characters!" and that's a little more on the subjective end. Some of it, like Teddy or Morgana, I think it's fair to write off as the Chibi Robo problem, but I can concede Tails and K. Rool could have at least had a full mask, but I think a lot of comes down to the fact that the creator of the character was on hand discussing what he'd like in a Mii Costume. Interesting to note is that of the characters that could have a full head (which, uh... Teddy actually kind of does?), it's Sega who gives the weird mascot hats the most. Maybe they just don't want the Mii representation of Tails looking like someone left a furry convention and ended up in Smash?
So as far as Geno goes, if the costume comes back, I think we're just getting the rat ass costume from 4, no upgrade, and this is because Square or whoever oversees the design of the costume thinks that the Mii face is good enough for him, in the same vein that it's good enough for Zero and good enough for Isaac.
EDIT: and the images broke. Just go look at the Goemon mii costume and look up official artwork and renders of Goemon.