I'm a big fan of dwarves, but separating them by how high of a mountain they lived inside was weird. It's apparently from Tolkien, but that doesn't make it necessarily something worth emulating -- we don't have a special stat block for treantwives, for instance.
I think mountain dwarves are a place where the 2024 books would have benefited by splitting culture off from species and background, as A5E and ToV have done, since their differences are more likely be cultural than "well, you live under an especially big rock, so therefore, you're physically different in these ways."
In my own campaign, there is a 3E-legacy split in the types of dwarves, with more urban mountain dwarves and hill dwarves who fit into Appalachian stereotypes. (It turns out there's a lot of crossover between hard drinking, family feuding and digging after valuable stuff in the earth among both groups.)
If I were going to mechanically represent that in 5E, I'd probably have the Grail (urban/mountain) dwarves have free proficiency with heavy armor. Barring that, I would have them suffer fewer penalties from armor generally.