This is slang use. She means, "It doesn't mean anything, anyway." Double negatives are not correct in English, so they can't be interpreted literally. People will sometimes speak this way to be incredibly casual or informal.

Double negatives may be rare in English, but they do exist. Here's one example: A. "Are you going to help that man?" B. "Of course I'm going to help him. He's my brother. I cannot not help him." In the example, "I cannot not help him" means "I must help him." And here's another example of correct usage of a double negative: A. "Don't worry. The fact that your boss yelled at you in front of all your coworkers means nothing." B. It doesn't mean nothing. It means he's probably going to fire me." In this example, "It doesn't mean nothing" means "It does mean something." "It don't mean nothing" can never be correct, on account of the use of "don't" rather than "doesn't".

"It don't mean nothing" = "It means nothing." Double negatives are interesting. They are bad English. They sound uneducated. OR, they sound like someone speaking under intense emotion and forgetting to use good English. Double negatives are common in spoken English. They might be used in a song to sound more authentic, more "real." Everybody understands double negatives. Using a double negative makes the meaning MORE negative. More "no-words" = stronger sense of "no." Sometimes people speak that way to sound tough--because they do NOT want to sound educated. These are examples of bad English. Understand them, don't use them. "You ain't heard nothing yet"--Al Jolson Means: you will hear a lot more. "I was scared, but a man says: 'He don't mean nothing; he's always a-carryin' on like that when he's drunk.'"--Mark Twain, "Huckleberry Finn" means: "Don't be scared, he doesn't mean it, he's drunk." "'But, Lord! It don’t MEAN nothing!' said the old flower-seller."--H. G. Wells, "Boon"