It seems to me that Christianity has more or less consistently had a problem with creating an idol out of its own religion, or at least, of the symbols thereof. An idol, of course, being anything that one worships in lieu of a deity. To me, the notion that a zombie nativity scene could possibly be "the equivalent of pissing on Christians everywhere" is at the very least subtle borderline idolatry. But of course that's my opinion. and I could be wrong -- even though I doubt it.
Actually, I think you're making a good point. If indeed it is true, as I have suggested, that displaying a zombie nativity scene on Christmas is so offensive so as to be equivalent to pissing on all Christians, I suppose, in a sense, that would be acknowledging that nativity scenes are so dear to Christians that they have become idols.
It is likely true that holding something, such as a nativity scene, so dear is indeed idolatry for the Christian. And that is wrong for a Christian to do. But what I am suggesting is there is a problem with the motives of the person displaying the zombie nativity scene, as he is failing to show respect for a symbol of something that is so dear to many people. It is not the zombie nativity scene I question, but the disrespectful motives of the person putting on the display.
In my opinion it is like burning the American flag in front of the houses of American veterans.