Making fun of people who are perceived to be powerful and successful, is more socially accepted than making fun of people who are poor and lack empowerment. The USA is wealthy, well developed and successful, at this point in time, Africa is not, so until this changes, making jokes about Africans will be seen as bullying type behaviour rather than harmless fun.
In the same way, it's acceptable to mock and ridicule the wealthy, but poor people are essentially a protected class.