>>530034016
I think it has more to do with the self-insert switcharoos than dehumanization.
The dehumanizing pronoun par excellence is it/its and monsters are very often "it" in in-battle narration even if they have an otherwise confirmed gender. Some kind of dehumanization there for sure.
I don't remember if humans are ever called "it". I was thinking maybe Muffet but she uses "they" too.

>>530034158
MK is a kind of mirror to Frisk so I wouldn't use them as a counterexample to the overall trend