There are degrees of racism? Wasn't that a binary thing? I understand that racists aren't created equal since some are filled with more prejudices than others but I think the same doesn't apply to racism itself. I'm not arguing that life is harder for people who are (or so we are told, I've got no data) the majority of victims but a victim being racist isn't being "less racist" after all. He's contributing to a vicious circle. Whilst I understand such behaviour from an emotional point of view, it's definitely not wise.
In most situations, saying "dumbblack" would be more offensive for black person than "dumbwhite" for white person. Especially in this case. It's like if you had a couple of bad experiences with the police, you could start saying offensive things about police officers in general. You could be accused of "policism" but in fact, it would be quite understandable reaction. When he said "dumbwhite", he was kind of "revenging" their racism, which is very different from saying "dumbwhite" just because he disrespects white people.
I generally think it's safer to frame in terms of the effect (which you mostly do) than motivation. Even if someone has no ill intent, and simply talks a certain way because they haven't really thought about the issues, their speech still does harm.
But it doesnt matter if its funny! It matters if its racist!
That's like saying that steak is food, but ramen isn't. Clearly, steak is considered a higher-quality food, but that doesn't negate the fact that ramen is food as well.
Due to the historical mistreatment of black people in America, it is more socially unacceptable to say something like "dumbblack"; however, "dumbwhite" is no less accurately described by 'a hatred or intolerance of another race'.
There's another definition, which is talk that's designed to maintain & reinforce the institutionalised power structure among races. Right now, if modern USA life was a video game, "white male" would be an easier difficulty level than "black male". There are statistically less problems for the "white male" group. Racist talk is talk that re-enforced that imbalance. This definition is harder for some people to accept because it means that you need to look at yourself and think about what power imbalances you might be benefiting from, and it means you can't just do s/white/black/g and make it just as racist.
So no, it's more racist to say "dumbblack" than "dumbwhite"