I figured Tyr especially is a mix of all human ethnicities, but in a lot of canon material it is discussed that every city-state has its own variation of Common. In some, they go so far as to distinguish them as different languages entirely (which I like, but find to be an unnecessary hassle for player characters to know that many languages). If the city-states are isolated enough to have different languages from one another, I would assume they have distinct ethnic groups.
Not to delve too much into the race issue, but I also have to assume there’s basically no white people in Dark Sun because (duh) its a desert, with the lightest skin tones probably being bronze or light brown. So far I’ve just been going off of what each real world culture looked like, but I’ll definitely check out the Ivory Triangle now that I know that info is there.
Basically, what I’m after, is when a player in my game creates a human character from a specific city-state, what will that human look like? In the 5e PHB they delve pretty deep into different ethnic groups and their traits, but there’s basically no information on what the humans of Athas actually look like.