One of my schizo theories is that monsters being able to use magic within the Dark World but not the Light World will become a plot point. The game doesn't really elaborate on why monsters and darkners have magical abilities in the Dark World, while humans assumingly do not.
This seems like a "who cares" plot point but the monsters in Undertale were all able to use magic. And every lightner monster (Susie, Berdley, and Noelle) had some sort of magic at their disposal down in the Dark World.