off the top of my head, gamma world, mutant crawl classics, Mutant (and its more modern version mutant year zero).
Mutants and masterminds depending on your definition of mutation, since its more like “superhero marvel mutants” than like “radioactive mutants”
Monte cooke games also has a 5e supplement called “arcana of the ancients” that includes mutation rules that are pretty good IMO.
I dont know if it will include it, but i am like 90% sure the upcoming mystic punks from exalted funeral is going to have something to do with mutants.

















fwiw, i think mearls usually gets kind of a bad rap online because he usually has some pretty questionable hot take answers to things on platforms like twitter. I’ve actually met him in person a few times and played D&D with him; He’s an extremely nice person, and a very good DM. i think he’s just not so good at his own personal PR online.