So I've been wracking my brain trying to think of an earlier story in any medium that used the radioactive mutant conceit. I can only think of one that kinda, sorta qualified. A pulp magazine hero called the Green Lama first appeared in 1940. One of his tricks was drinking a solution of radioactive salt. This temporarily gave him the power to shock bad guys unconscious simply by touching them.
But that really isn't the same thing as a mutation into something monsterous or superpowered. Was this Superman story the first time this happened? I just don't know for sure. There might be a B-movie or something from Astounding Stories magazine that covered this ground first. Or there might not. It's one of those questions that there's really no effective way to research.