I sincerely doubt it is an Undertale rep, and I can tell you why.
I'm a massive Undertale junkie. Heck, I've been a fan of Toby's work before Undertale even happened. I'd like to think I know Toby's ideals pretty well enough, and I can safely say Undertale reps in Smash would be an affront to them.
I could see Toby certainly wanting Undertale featured in Smash in a small way, perhaps via spirits, but in terms of a full, playable character, I can't see it. Having Undertale represented by so much as a corporate merchandiser was enough to turn him away, for he'd shot down those opportunities and went to Twitter over them. He prides Undertale in being a labor of love, not a 'franchise' or anything 'corporate'. Undertale being its own thing in Smash, being a franchise represented with a character, doesn't feel like a very Toby thing to do. He loves Smash, and he even made jabs at it in Deltarune, Undertale's sister game, but to go that far into the corporate rabbit hole with it is against everything he's publicly said about Undertale's future.
Plus, Shantae, the longest-running indie franchise under Nintendo's belt, being just a spirit? Shovel Knight, a Kickstarter success arguably bigger than Undertale, being an assist trophy? Yeah, I just don't see it happening.