Some comic-book events feature a black Superman, and that raises questions about whether Kryptonians coincidentally match every ethnic group on Earth

Not exactly, but they have dark and light skin. Krypton's red sun is a big deal in Kryptonian health. Kryptonian communities living closer to the equator would probably evolve higher melanin the same as humans did. Actually, light-skinned Kryptonians require a bigger suspension of disbelief than dark-skinned ones do.

And obviously the racial dynamics between dark and light Kryptonians wouldn't be the same as on earth. It would depend upon factors like the shape of continents, the location of arable soil, of large herbivores and of predators, and of minerals.

Fun fact: swords made out of meteorites are a common trope in Japan, because Japan has poor iron and big mountains.