I'd only ever heard the term in some old song lyrics, but a look online suggests it's gained popularity recently.
Here's more from 'Urban Dictionary': A girl who is very independent, not relying on other people & fights for what she wants. A Material Girl also tends to like expensive and luxurious things. She is also a girl who knows what she wants and will do anything in her power to get it. They tend to show confidence and egotistical behaviors even though they are not really like that.
I'd say it's negative but maybe some people would disagree, depending on their outlook on life.