Well, first of all, the movie is a social satire, and similar to films like Blazing Saddles, you're going to have to sift through piles and piles of unrealistic foolery to get at the film's underlying message. Not unlike other black people within the United States, Spike Lee is disappointed with the modern representation of black people within television/movies, as well as the current state of the industry as a whole.
Mantan is basically an incredibly exaggerated representation of actual black television, which outside of a select few shows, is basically restricted to the comedy realm of the medium (Fresh Prince of Bel Air, Family Matters, Sanford and Son, etc). There is certainly nothing with these types of shows, but a problem arises when comedic roles make up the overwhelming majority of your ethnic group's depictions, considering characters within comedies tend to be archetypes and hardly what anyone would call flattering. In the older days of Hollywood, the black American's role in film outside of servant roles was the comedic relief, people like Steppin Fetchit or Mantan Moreland (!), who basically embodied socially held stereotypes about black people for a cheap laugh. And this is the coorelation that Spike Lee is ultimately trying to get at, the fact that while black representation is leaps and bounds better then it had been prior to the 60s, it hasn't actually progressed that far from its origin.