Implying Nintendo would go through the utter licensing nightmare that is dealing with the Mouse, spending God knows how many bank's worth of cash, all just to acquire a measly Mii costume that would do nothing but piss off fans.

Sans is a different matter entirely because he's an indie character. Any indie dev would BEND OVER BACKWARDS just for the chance for their character to have so much as a mention in one of the most esteemed crossover games of all time, so the licensing fees and whatnot aren't even comparable.

tl;dr negations for Sans is like a sardine talking to a tuna. Negotiations with Disney would be a tuna talking to a MOSASAUR.

I would be more shocked than if he was an actual character, because this scenario would mean that an unprecedented licensing deal between Nintendo and Disney was made, and Nintendo only used it for a costume.