As far as I understand, in One Punch Man all monsters are mutants. According to the wikia
humans naturally become monsters from a desire to transform into something else through their bad habits, a complex or a burst of dissatisfaction with their regular selves. They begin their transformation due to their environment that triggers an abnormal cell reaction
But how about heroes? Some of them are indeed mutants ie, Zombie man. Tatsumaki and Fubuki ESP powers have to be mutations, dont they? How about the others? We know Saitama got the strenght he has for breaking his limiters so it wouldnt be a mutation, but do the abnormal strenght, resistance, speed, fast healing etc some of the others display (Tank Top Master, Metal Bat, Flashy Flash, Watchdog Man, Superalloy Darkshine, etc.) is the result of mutations or not?