Assuming it isn't bound by the "everything in D&D has a home here" rule that seems to apply to published settings, then for that setting I think I would be tempted to at least imply that this is some sort of post-apocalyptic, possibly-alternate, Earth. As such, I'd be inclined to rule that it is
humans who had retreated to the tops of mountains, and that they are the only playable race.
I'd be inclined likewise to either remove arcane magic entirely (and use psionics in its place), or handwave arcane magic as a new and unexplained result of the disaster that occurred. Likewise, I'd probably posit a largely-undefined religion based around "The Source" that empowers divine magic, and that has only just started to respond in that manner.
I would also seed the world, both in the mountains and the covered lands, with some remnants of old technology, but it is old, oddly-specific, and largely understood. So those ships that the PCs use exist, and people know how to maintain and repair them, but they don't know how to make new ones (or, really, how they work). Likewise, I would have various radio signals going around, mostly in the form of odd number stations and other bizarre transmissions - but have the radios be large, unwieldy, and solar-powered. But no computers, not much modern medicine, and lots of other gaps. Oh, and there are probably some nuclear reactors still running somewhere under the clouds... (A lot of that leans into "The Long Tomorrow" by Leigh Brackett.)
For the mutants, I'd tend towards beasts and abominations, and away from outsiders. I'd also tend away from using intelligent races, though I would almost certainly have lots of different tribes of mutants of one sort or another - but underneath the strange appearance these would also be human. That said, I might well have duergar and drow civilisations on the surface of the world, with the implication being that they were down below all along and have now migrated. Just because.
One other thing: in contrast with most post-apocalyptic settings, I would be inclined to make this one hopeful in nature. Yes, there has been a disaster that ruined the world. But people survived, they're making a living, and things are gradually getting better. In fact, the biggest opposition to things getting better are various factions opposed to it - leadership councils that have prospered in the new normal and don't want to give up their new power, doomsday cults that want to complete the job of wiping us out, and the like. Oh, and from less-successful mountaintop cities somewhere in the North, young men and women would set sail and go raiding.
As for a name, I'd be inclined to reflect that the cities lie on the crown of the mountain with the problem posed by those raiders, and call it something like "The Corona Vikings". Or maybe not.
