McFarlane has rightly equated blackface with redface--i.e., dressing up as a phony Indian. Even as a "joke," this makes a valuable point seldom seen in the mainstream media. Almost no one gets that one practice should be as socially unacceptable as the other.
Of course, the joke was subtle enough that viewers still may not get it. And I'm not sure McFarlane believes believes both practices are wrong. He might claim he was satirizing political correctness and thinks both practices are okay.
After all McFarlane has given us The Cleveland Show with a white guy voicing a buffoonish black character. And Mort the Jewish pharmacist is about the only ethnic character on Family Guy. Otherwise it's pretty much a bastion of whiteness.
Proving McFarlane's insensitivity, he has Quagmire claim he's part Japanese in this episode. This provides an excuse for several racist "jokes" about Asians. And in the latest degradation of daughter Meg, he has Chris and Meg commit incest by kissing and groping each other in a closet.
For more on the subject, see Nanookwaffe in Family Guy and Rapacious Cavalryman in Family Guy.