It's top on metacritic and basically has the most upvotes a single Steam game has ever seen compared to downvotes.
You sure about that?
why would you use skyrim as an example skyrim is pretty forgetin stuff compared to undertale
skyrim is good and enjoyable, (even more so with mods) but only as much as a "mindlessly chowing down a bag of chips" or "call of duty" level.
also the fact that most of your criticisms of the game boiled down to needless nitpicking (no footstep sounds? really? might as well have thrown in "too much water" while you were at it) i wouldnt say you're an expert game reviewer yourself.
undertale wasn't exactly an ambitious project. it is actually a pretty simple game at first glance. it used a strict rpg formula and (assumingly) purposely put in some aesthetic 'flaws' to make it reminiscent of classic rpgs (it also probably didn't have an AAA title budget. i cant remember how much money it got from kickstarter but it wasn't really that much). the reason why the story's narration and story is so enchanting to so many people, however, is because it blends in incredibly unorthodox things into the standard, almost 'rpg maker' level of formula. this is also pretty effective for criticizing the usually monotonous rpg formula, which im pretty sure was mentioned before.
the actual gameplay is fundamentally better than most games in the rpg genre for the same reason i listed above. at first even the concept of its basic skill-based bullet hell formula for battles struck me as a unique and interesting take that blended well with the concept of not hurting anyone/having to kill everything the way a traditional rpg would have you do. but stuff like the muffet boss fight, and the god damn SANS boss fight really hammered that in.
maybe it really is people baffled by its unorthodox and unique style that caused it to have better ratings than games like mgs:v (an unfortunately unfinished game, actually). but even then, it kinda makes mgs:v look like a bland formula in comparison. it doesn't bring that much to the table, other than sticking to a formula and being really forgetin good at it. doesn't make it a bad game, just not something that makes people go nuts in the way undertale has. people crave new stuff, and when a game delivers that in large quantities, a lot of people are gonna like it.