But to put it simply: segregation is enforced. This is not that. You're making sweeping assertions based on a very limited data set.
And you aren't? I'm arguing from logic. I've explained that, normally, the metropolis of a multicultural state would have a diverse population, at least in the general public if not in the ruling elites, because there are strong political, cultural, and economic incentives to draw people there from all over. I cannot see a way that such a metropolis would be completely devoid of diversity
unless that segregation were enforced, unless other groups were actually barred from entering it.
Also, again, the
Ruling Council of the entire civilization (or at least the pacifist New Mandalorian faction that controlled the system at the time) was portrayed as monoethnic. That's pretty hard to handwave away. If Mandalore's population is based in clans, then shouldn't all the clans be represented on the council? You can't brush aside the implications of what you're arguing just because you don't like them. If your thesis is that different ethnic appearances of Mandalorian characters represent different clans, then it follows that the New Mandalorian Ruling Council consists of members of only a single clan, or a single group of closely related clans. And that means it's not a democratic or representative government, but just an oligarchy. I'm merely following the chain of logic from your own postulate to the conclusions it suggests.
Which is why, as I said, I'd prefer not to take this as literally as you always seem to want to with
Star Wars. It's a television show. They made a bad design decision. They made a mistake of defaulting to white, and since then they've been doing better at designing and casting characters more diversely. If they went back and made these episodes today, maybe they'd make the population more diverse.