One of my headcanons is that benign mutations are widespread among wastelanders but by no means universal but still common enough to be accepted as normal or unsurprising. This is something that just felt off to me in Fallout because there are lots of mutations in creatures that have changed them visually and physically but humans themselves seem mostly identical to pre-war humans.
One thing I'm a huge fan of is mutants that are mutated humans but aren't just Supermutants or Ghouls, precisely for this reason. I like the idea that there's lots more diversity in how humans have responded to the apocalypse than what's shown.
Fallout 2 does this pretty well in a few places: Lumpy in Gecko appears human but is his own thing completely immune to radiation and was kicked out of Vault City because of it. Moreover the Slags are pretty cool, these underground people who frighten others in to creating myths about a haunted farm.
To it's credit, Fallout 3 also heavily implies that the Vampires are mutated humans somehow that have an addiction to cannibalism. I have to say I don't like this as much as the Slags in Fallout 2, where the mythology around them is just that, and really they're just another benign group of humans trying to survive, but as far as Fallout 3 goes, I quite like it.
The Trogs and Swampfolk would also be good examples of this, though I have mixed feelings about the latter. On the one hand having a hostile group of rural people so utterly othered by the narratives that they're effectively elder god worshipping maniacs who shoot on sight is very fitting with the Lovecraftian theme, but on the other Bethesda relies too heavily on "Another group of enemies" as a crutch.
Within my headcanon I would fit some people like Lanius as having a rare mutation that puts him above the common man in not only height but strength as well. I mean have you seen Lanius? the man is an absolute monster who stands at least 7 feet tall and runs 80% faster than the player while also having significantly more fatigue points than a normal NPC.
I'm not sure how I feel about Lanius specifically being a mutant.
Honestly, I like Lanius because he's so shrouded in mystery. The idea that he is just this bulking monster who could crush you in an instant is far more terrifying then him being a radiation or FEV construct.
I don't really mind semi-legendary characters who have abilities that seem unhuman. Joshua Graham's survival is never explained beyond "God saved him", and I'm happy with that. Legion is filled with this legends of characters doing superhuman things, and I'm happy with that being the way things are, without any attempt to explain it.