Show us the cartoon - then we'll tell you!
Racism, class envy, etc. etc. all these are valid criticisms of McGruder and Boondocks.
The most valid criticisms, however, are:
1) It's not funny! Hurt white/Pubbie feelings aside, there is rarely any humor in the strip (OK, I glance at it despite myself). It's simply bitterness, even hate, expressed through cutesy characters.
2) It's not original! It's Bloom County in blackface. The pregnant pauses (often a whole panel is given over to two characters staring at each other) is a huge Berke Breathed ripoff. One laid-back character chiding another for getting worked up over the issue du jour - again, a Breathed staple.
Of course, you can dig deeper and cite Schulz et al but at some point all cartoonists will hide behind the 'influence' tag and deny they are ripping off the works of others.