Chaos magic (which exists, dammit) is slightly different in that it's drawing upon the "natural" power of chaos that exists in all universes. Like the speed-force, only it's in every universe, and somehow makes a lot more sense in application.
If you're talking about a mutant APTITUDE towards magic, then what would be the point? To make up a random name as an example.....
Gene Genie: "Ha! I have the ability to learn the greatest spells in all the multiverse, due to my mutant genes!"
Doc Strange: "Uh... so do I, in my 80s stonewash jeans, and Sentinels aren't trying to kill me on sight."
If people WITHOUT a mutant gene can reach the pinnacle of magic, why would a mutant aptitude be any use, except as a targeting system for Sentinels?