That's more of an example of "laughing it off," as opposed to "putting yourself down." When a girl takes a shot at your ego, and you come back with something like that, you're essentially telling her that you were unaffected by her attempt to belittle you. In fact, you were so unbothered by it that you're willing to make a joke about it. It implies that you're secure knowing that you DO get girls, and that you're pretty much impervious to her attacks.
Picture the alternative: acting all pissy, and trying to defend yourself. Not good. It's much better to show her you don't care what she thinks.
"Putting yourself down" is when you take shots at yourself, unprovoked, out of the blue. It's different from "laughing it off" in the fact that you appear to believe what you're saying; that it's not a joke. It implies low self-confidence and low self-esteem.
Being able to take a joke, and the occasional jab to your ego, is an awesome skill to have. You don't want to be one of those assholes who takes himself too seriously.
Of course, what you say isn't nearly as important as how you say it.
Just my two cents.