Zardnaar
Legend
With some imagination a Dm may find a solution to include any race in its seting.
Tiefling may look human and explain racial power by a possession, family tree or a like.
i like your mutant explanation, it can also be arcane experiment that has go wrong, a curse, bug in plane travel. It can be cool to be a unique racial member.
Uniques fine it's when you get a whole party.
Athas has mutants so any race as a on off can work, I would just ban things like warforged.