I would say being the "face" of the franchise is definitely more important relevant then being the main protagonist, Cloud is the "face" of Final Fantasy, Ryu is the "face" of Street Fighter. Obviously, most of the time the "face" is just the main character of the game.
However, there are certain cases like Sans; where a major character is much more recognizable then the main character. And even then, Smash tends to focus on getting them in first.
Both Pikachu and Mewtwo got in long before Red. Elma is much more requested then the "protagonist" from Xenoblade X. KOS-MOS is much more requested then Shion. Even besides that you have trophies like Rathalos that got in before Monster Hunter.
So yes, I think having the biggest character from Undertale represented in Smash as a DLC mii costume pretty much deconfirms any Undertale character.
I already adressed Pokemon Trainer not being possible in older versions of Smash. Also, last time I checked, Elma and KOS-MOS aren't playable characters currently, so fan demand doesn't mean anything if it doesn't currently have actual results. Whether our whims get catered to is entirely up to Nintendo and Sakurai's own whims.
The internet at large very much treats Sans as "the face of the game," yes. However, Undertale's
official marketing material doesn't treat Sans as especially more important than Papyrus, Toriel, or Flowey as "mascots" of the franchise. If any character other than Frisk got in, I could easily see it being one of the three I just listed. Especially since they all fight in neutral and pacifist routes, wheras Sans is heavily associated with the darkest route in the game at this point, and will literally only fight under those circumstances.
The main reason I think the human might be more likely than the others I listed, other than the Protagonist Trend, is simply that they could probably have the broadest potential to represent multiple game mechanics and parts of the game in a single character, and so could have a lot more creative moveset potential than "the skeleton has bones and lasers" or "the goat uses fire."