Thats the thing, no mutations, not even in mice that live in burrows and have like a generation every two seconds. They even did a DNA study by comparing species to the ones not from that area and found no differences.

But the main thing they looked at is cancer rates/signs (ionising radiation causing random mutations resulting in cancer, not superpowers), thats why the mice focus (but the fauna there is thriving, the biggest are deer).

The radiation causing mutation is very theoretical in the sense that the chances if it happening and leading to problems (and DNA corrective measures) seem to be low in the sense that radiation levels needed for that will sooner cause tissue damages too (which ofc is a thing that happens & kills).

There is still a lot we don't know bcs there are so few nuclear accidents (and bomb test) sights to study, but the levels how we defined safe is way on the conservative side.