I really think Chan Ho Yin should logically be a mutant. He's based off a character from the comics who is a mutant and his powers were
almost completely unexplained.
Chan Ho Yin (Chinese: 陈浩然) was a super-powered individual on S.H.I.E.L.D.'s index who developed pyrokinetic powers. While working as a street magician, he was recruited into the Centipede Project by Raina, adopting the name Scorch. However, Centipede betrayed him and proceeded to experiment on...
marvelcinematicuniverse.fandom.com
Ray Johnson, who you may know as John Wraith from X-Men Origins: Wolverine, was referenced on WandaVision. He's a mutant character but we know nothing of him in the MCU.
Ray Johnson is an individual whose medical results were held by S.W.O.R.D. Ray Johnson underwent a medical examination conducted by S.W.O.R.D., the results of which were stored in the agency's files. While hacking through these files in search of answers on then Director Tyler Hayward's...
marvelcinematicuniverse.fandom.com
Of course, Sony's Spider-Man Universe has already introduced mutants into its continuity with Shriek. Madame Web is also a mutant. There's no reason to believe they aren't mutants so I'm assuming they are.
I really don't think we need a big explanation for mutants in the MCU. Just introducing them naturally like any other character is fine.