SSGuy
Smash Champion
The face on the Vault Boy or Cuphead's face are just not possible to recreate as a Mii. Which is why they are created the way they are. There isn't any special treatment Cuphead gets over Geno just because he has a full face but the community has in turn made it this hidden rule to gatekeep other Miis like Geno or at least do it in a sense to undermine Geno's representation. It's not just exclusively him but it's definitely been an ongoing thingI think whether a costume has its face covered or not depends on what the character's most recognizable features are.
Sans' cheeky grin, Vault Boy's 50s art style and Cuphead's... Uh, cup head are what defines them, whereas Geno's most iconic features are his hat, cape and gun and Shantae's are her clothes and whip-hair.
(Granted, I don't think the author of this comic was being malicious but it's definitely an example of how just plain wrong information can shape a community and the content it creates)