It appears the humor and frivolity was for the purpose of lessening the inevitable backlash from those who get upset when they hear the word "nigger". The movie had a bunch of really subtle parts meant to counter any appearance of blacksploitation as well, like the half a second shot of Django's huge bulge in his pant leg and the fact that the only words he couldn't pronounce were "gang" and another similar one I don't remember. Also, the purpose of many of the jokes was just to make the whites look idiotic.

The movie probably could have been more serious if it was done by a black guy instead of a white guy because of idiots like Spike Lee who instead of jumping from a bridge like he should, he cries racism when he really just has no fucking clue what's going on.