>>719012739
In Undertale, monsters are very explicitly made of a handful of matter held together by magic, which is why they disintegrate instantly. Deltarune's light world very obviously has no magic whatsoever (don't ask me how fire elementals and other physically impossible monsters exist) yet only monsters have the ability to cast magic in the dark world. Organic-type monsters, aside from their dark world abilities, are physically similar to humans and can get sick or injured in the same way; this implies that they have blood and aren't like Undertale monsters. Susie is also far stronger than Kris which flies in the face of Undertale canon. Of course, she could be a monster/human hybrid like some have speculated, but there is nothing hinting at that instead of "monsters here work differently."
We don't know much past that. Kris and Susie's recognition of the significance of dust in a snowglobe implies that monsters all still turn into dust rather than have to be cremated, but this might not be at the point of death like in Undertale. Anything to do with monster souls is also speculative.