If Hero's anything to judge by, then I'd assume they'd name Frisk "Fallen Child", though it'd honestly sound a bit awkward in the announcer's voice. But yeah, Smash usually hasn't bee too sensitive about spoilers, and if Undertale did get into Smash I can see Sakurai pulling concepts from all across the game, especially for Frisk's stage and music, along with the weapons you can pick up. There could even be alts based on the weird armor you can pick up such as the Tutu.
I'm still wondering, when it comes to Bethesda reps how come all of the attention has gravitated towards Doomguy? With other companies with likely candidates the attention is pretty even (for example, between Lloyd and Heihachi), whereas here it's honestly very skewed. I know why Fallout isn't getting much consideration, but why exactly are people sleeping so much on The Elder Scrolls? It's Bethesda's home grown franchise (while Doom was acquired), it's 25 years old as of this year, and has had good integration with Bethesda and Nintendo's collaboration (Skyrim featured prominently in Switch reveal trailer, BoTW stuff in Skyrim Switch along with Skyward Sword-esque motion control options, Skyrim serving as major inspiration for BoTW, etc), whereas for Doom it's mainly ports, and ports that aren't Nintendo exclusive (from what I read they're multiplatform releases; though I do know that a lot of them were originally SNES games). Doom does have a more consistent and iconic character, but I'm just curious as too people's opinions and why Bethesda speculation has gravitated so much towards one character, and why there's so little discussion regarding The Elder Scrolls. If Bethesda's gonna represent themselves as a company, I don't know why they'd choose an IP they acquired over one they made themselves and that made them so successful in the first place.