Super Smash Brothers Ultimate - Interesting details you may have missed while hysterically laughing during the reveal itself.

Also, if you pay attention, you will realize that the choice of Pictochat's "Now entering" on a black background with sans right above is a reference to Undertale's battle interface.

…as for why the Mii costume is shown on Magicant afterwards… well…… who knows, really.

Because UNDERTALE was inspired by Earthbound?