Shameless nitpick: There's nothing wrong with the logic that "radiation causes mutations, so more powerful radiation causes more powerful mutations." If you expose yourself to a thousand rads, you will get a heck of a lot of mutations. The logic breaks down when you expect these mutations to give you super powers, rather than a big mess. It sounds like you've got the superhero logic backwards: people did not look at evolutionary theory, understand it incorrectly, and then hypothesize that superheroes should be an expected outcome. They first made up the superheroes, and then looked for anything which might plausibly explain them.