I don't think it is really proportional to the demographics of the area in question..
We go to all of the NARBC shows, and many of the Repticon and other shows, too. I have been to shows in Chicago, Anaheim, Philadelphia, and lots of other big cities, and all over Florida. Even if the shows are in some part of the city that has few minorities, I think they would come to the show if they had any serious (or even semi serious) interest. The NARBC shows draw people even from out of state.
Because I have always been interested in other cultures (we have had lots of foreign exchange students back in the '90s, from all over the world), I make it a point to talk to people at shows who either "look different" or "talk differently" from your "typical 16 - 50 year old, middle class, white herper" (that would describe MOST of my customers). I am always curious to find out what their interest is, if I am not too busy at the show to just have some conversation. And curious as to what country they are from, if they have an accent. Unfortunately for our hobby, MOST of the minorities attending turn out to be either employees of the facility on their break, or (actually, fairly commonly) one black member of a white family that dragged him or her along with them for a day at the "reptile zoo". I have met a few with a real interest, but very few.
I guess if we do meet any black or Hispanic herpers, we should try to get them to get others involved. I think it would be helpful to our industry / hobby to have as much diversity as possible. But I believe it is somewhat of an uphill battle. As Janine said, different cultures have a different outlook on the place of pets, and the type of pets, that are suitable for their families. But with the exposure of Internet, TV, and people going into schools with herps, that could change for the next generation.