Whats the difference between a prison and a zoo? Zoos have lots of animals, prisons just have monkeys.
Are you implying that prisons are only full of certain types of people?
Mexico is the most dangerous place on this planet, most of people are either criminals or drug dealers or work for drug cartels.
There are an estimated 10+ million prisoners in the world. Making any implications about race inside those prisons is quite dependent on the country you are basing the joke on. Rather than trying to joke about the topic, check out some books or tv shows on prisons across the world, it might shock you!