Gonna make your quote subject for friendly, good-spirited-fun-poking conversation.
"It ain't braggin' if you can do it."
Sorry, but as "cool sounding" that is, it doesn't make sense.
People don't call anyone a brag when they're claiming to do something remarkable that they "can't" do. They call them a "liar."
And if someone is talking up something excessively, just because they "can do it" is not a valid defense from being labled a bragger, otherwise there'd be no legitimate application for the word, and as such would have no meaning, no reason to exist, and would NOT exist.
So, while one manages a remarkable feat they don't deserve being called a bragger, but if they do excessively talk it up, then they are.
Am I right?
So the only accurate way to express anything similar to that quote would have to be "it ain't braggin' if you don't talk it up."