OK let's run this up the flagpole.
Undertale takes care to establish a clear visual parallel between Sans and Frisk. Sans's first appearance is in complete shadow, and he's right behind Frisk and the same height. It's almost as though Frisk is forced to confront their own skeleton—as if Frisk were doing the equivalent of looking down into their own grave.
Except, Frisk is only doing it because they're being directed by a human player. Fox and the rest of the Undertale team has arranged this visual parallel for the player's benefit. If Sans is a memento mori, then it's for the player to see. It's the player who is being invited to think, "Some day, I'll be a skeleton. (Probably.)"
Our bones can remember things that we've consciously forgot. They're with us most of our lives, and the inorganic salts that give solidity to bones are the most permanent part of a human body; so much else is constantly being sloughed off and replaced, but our bones record facts about our lives—how nourished we were, whether we've been seriously ill, whether we've been severely injured. Our bones remember our traumas...and if we're fanciful enough, it's possible that our bones remember our sins.
At any rate, if there's any part or aspect of being human that's best suited to sit in judgment of the rest of one's existence, I suppose it would be one's bones. ~Chara