I've noticed the lack of Asian characters in the world of Firefly too. I expect there's some backstory, like China lost the big war but a lot of their culture seeped into the culture of the West (either before, during, or after). I also don't think it's a coincidence that the only three male African-American characters (Book, the bounty hunter, the operative) have been highly trained bad-asses, highly literate and philosophical, and worked for the Alliance.

Also, both Zoe and Anarra seem to have vaguely Asian features. Or maybe that's my imagination.