>>141525 (OP)I'll sperg a little on the topic.
Toby Fox didn't try much with Undertale as a principle; When Deltarune was announced, he once explained that Deltarune was the game he planned back in college, while Undertale was his way to test out if he can make games at all. He sucks at games, visibly - but it didn't deter him. Honestly his logic is weird.
He also had 2 artists - Temmie Chang and some other starting with G (I forgot the name); The second artist drew sprites for the battle encounters, mainly, while Temmie handled everything else. Foxenstein himself drew some things for the game - so as you pointed it, it is indeed done in different artstyles. Some of the monsters you see in-game were also Tumblr OC's that fans submitted during a campgain where anyone could send Toby a design.
Also a quote about the player sprite, which really shows Toby's attitude: "Everyone says the protagonist's sprite looks terrible. That's right. If it looks too good, you'll have a different expectation.". This is from the game's artbook. Him and his new team of troons (geg I'm not lying) are now working on Deltarune in the same vain. I can attribute the area design to this as well - the setting is underground, obviously it would be dark, but detail is omitted not as an artistic decision but because the dev didn't bother.
Toby's business model is pandering; Tumblrsluttas and children didn't really care that the plot makes no sense, or that the game looks weird, they liked le heckin' lesbians, feel-good message and "fandom" elements.
I never really felt that Undertale actually unsettles, it just feels like an unfinished build, or concept build of a game that would still be developed.