To order off the top of your head means to do it from memory or without much consideration, but in this context it talks of speakers incapability of ordering at Denny's after giving it some thought.
Consider the statement "You couldn't order right in a dennys after giving it some thought and consideration .". Based on this "You couldn't order right off the top of your head in a dennys." a true statement? Yes, no, or I don't know?