Not sure of the question, as i don’t think there is racism in mountain biking, there may be people with racist views, same as every walk of life with every ism being seen.
Through 20 odd years of doing this i don’t see mountain biking in the way i’ve been involved as being racist, it’s not exactly a closed environment that requires some kind of entrance criteria or acceptance, you buy a bike from any shop, get some skills, join a group via many forums (facebook, clubs, work, friends, etc, etc) and go out into the publicly accessible areas, you can ride alone, with a mate, or with a group.
The question as to why certain groups in society are under represented in this activity is a question for those groups, and it’s a wider grouping than just a colour.