In light of the whole “Sans undertale in Smash” thing, let’s get one thing straight here
That ISN’T Sans. That’s a Mii Fighter.
That’s one of them dudes that look like people wearing a costume of Sans fighting for him.
I don’t mean to burst anyone’s bubble regarding this, but I actually think it’s super in-character for Sans to be a Mii Fighter costume.
Like, the guy couldn’t even be bothered to do any actual fighting himself, so he got a Mii to do it.
He got YOU to do it.
Sans just throws you his clothes and a mask of his face, and says, “here you do it.”