nah, it's just a costume for Mii Gunners. It's funny though cuz for one; it's probably the one Mii Costume that actually looks pretty much like the actual character (especially with the mask). Not only that but they bundled the costume with a Special Remix of Megalovania, which is pretty much unheard of. Needless to say, the community wouldn't stfu about it for a few days. |D;;
That said, a part of me is grateful they didn't make Sans an actual character. If Undertale was to get any representation then it should be from Frisk/Chara, and as far as Indie reps are concerned I'd rather see someone like Shantae be used instead.