I don't remember her, but the actress who played the Romulan ambassador Caithlin Dar is Chinese-American. There have been very, very few Asian (or Latino) actors and actresses cast as aliens in Trek in general.
Also, for the Cardassians, Legate Broma (from the last two DS9 episodes) was played by a black man, Mel Johnson Jr. That said, they caked him with the typical Cardassian grey and gave him the typical slicked back straight hair, so it was fairly hard to tell.
The Toba supereruption hypothesis is now frowned upon.
Regardless, genetic diversity is not the same thing as phenotypical diversity. Yes, it's true that we are far more genetically related to each other than different groups of Chimpanzees are. This is to be expected considering our most recent common ancestor (discounting archaic admixture by Neanderals, Denisovans, and other groups) was less than 300,000 years ago. However, despite genetically diverging for a longer period of time, different populations of chimps don't show the same sort of dramatic surface variation (height, skin color, eye color, hair color, hair texture, etc) that humans do. Basically chimp genetic divergence has been essentially neutral drift. In contrast, the comparably minor genetic differences between different human population groups - including those we use to identify different races - are very noticeable in terms of phenotype because different human groups underwent natural selection in the relatively recent past.
Extrapolating this to Klingons and other Trek aliens, genetic diversity is not enough - you also need to have different population groups undergo selection for very different physical forms. For the most part the modern human "races" took shape over the last 50,000 years (with the last 10,000 in particular being important). Big physical differences like the Klingons show would require substantially more time. Or meddling by the Hurq or something. Though admittedly, Trek has always been very dodgy in its understanding of genetics.