Anyone complaining over the use of "retard" or Robert Downey Jr. in blackface is a complete moron. Fact.
The whole point of the film is the OTT nature of Hollywood. Ben Stiller's character plays a mentally disabled man in a movie called "Simple Jack" in order to gain credibility, while Robert Downey Jr.'s character is so self-obsessed that he honestly believes he is so talented that he can play a black man in a film and nobody will have a problem with it.
It's parody, pointing out how stupid and ridiculous the characters are. The people protesting don't understand comedy or what the film is actually trying to say.