I know /vg/ is mostly just gay porn now but I thought I'd share my thoughts.
toby places an extreme importance on smiles in UT/DR, and if you look closely you can see that he made almost all of the smiles unique and recognizeable, an aspect of their character.
the most obvious exception to this rule is the dreemur family who all share very similar smiles, and this is for obvious reasons; they are all part of the same family, part of the same species. the same smile.
there are thoughever two other notable exceptions:
1. flowey's evil grin is extremely similar to papyrus' smile, the biggest difference being that flowey's is viewed from the front, but papyrus' is a partial profile. this is notable because flowey regularly immitates other character faces, and papyrus has an established "special friendship" with flowey, flowey having a particular liking for him. maybe there is more to this grin than we know?
2. sans' smile is EXTREMELY similar to... the smile in the TV static in chapter 2, dreemur levels of similarity even. this style of smile is only seen in two other places: the chapter 3 icon (which makes sense), and FRIEND's pixel sprite. while it is commonly assumed that this smile is tenna, there are a lot of extreme differences between tenna and the static smile. tenna is not depicted with that kind of smile, even in his concept art. it's a smile connected to tenna, but not "tenna's smile". but if not tenna, who's smile is it? the greater plot of deltarune, with FRIEND, the bunker, the shadow crystals, etc. all go back to a very sans-related character: gaster. the smile on the TV could genuinely be our first glimpse of gaster's true face. a mark of his behind-the-scenes manipulation of the plot.