We throw around cracker, kkkracker, Yakubian bleach demon, mayo, and joke about mayocide a lot and pretty much always in relation to white Americans doing something horrible or obnoxious. I don't think most people are using it in an essentializing way, which sounds weird, but I think we're using it for "White" in the sociopolitical sense, to highlight the power of white supremacy in politics by using pejorative words in a way similar to how white supremacists use racial slurs. So the joke isn't that all people with pale skin share common traits due to superficial appearance, but rather a way of mocking white supremacy and white American culture by juxtaposing it with white supremacist language. Which ties in to racism being systematic oppression and white people not being subjected to systematic oppression anywhere, thus you can't be racist against white people. Prejudicied? Bigoted? Sure, because those are individual qualities. But racism requires that there be a system of oppression in place that oppresses the group, and that doesn't exist for white people.