I think the right knows they're being silly when they act offended by "cracker", and so does everybody else. When I was a dumb highschool kid I recall having a conversation with one of my dumb kid friends about this, and laughing at the word "cracker" because how could it possibly be offensive to us white people? It signifies no shame or perceived lower status, unlike other slurs. Libs get this even if they pretend not to.
edit: just to add, I think all you need to do is point out that anybody pretending to be offended by cracker is really just mad they're not allowed to use actual offensive slurs, not at the word "cracker"