If a big enough asteroid hit the moon, would it be off orbit?

What do you mean, "off orbit?" An orbit isn't a balance beam, where the merest push will send it careening into outer space or onto the Earth. Hollywood aside, there is no such thing as an "unstable" orbit (except for those that are actually within a tenuous layer of the atmosphere and suffer air friction). Every asteroid that hits the moon changes its orbit, even if only by an imperceptible amount.