It doesn't stand to any reason why Namor would look Asian. His father is Irish or Scottish-American and one would think that Atlanteans would resemble people from a continent bordering THE ATLANTIC OCEAN, IE not Asia. So the fact that some artists are geographic ******s shouldn't effect the casting.
Maybe Atlanteans could have a Native American look, although that still would be kind of tricky math if you would have to say that Atlantis was established AFTER Asiatic people crossed over the land bridge and all of the Americas.
It's just easiest to make Atlanteans look some combination of Western European, mediterranean or West African.