EDIT–need to make my math add up a little better
Here in the New Mexico, US, there are quite a number of Hispanic MTB riders, but any other ethnic groups, except we caucasians are very few in number.
If I was to guess, I’d say the ratio is caucasian/60-65%, Hispanic/30-35% and all others the balance (which isn’t drastically different than the ethnic population makeup here in New Mexico)
As an aside, the ratio of men to women MTB riders here is probably men/85-90% and women the balance, based on who I see out on the trails every week.
I think that the cost of MTB is most certainly a factor, at least here, as New Mexico is, historically, more of a rural ranching/farming state except for the 4 largest population centers).