Well, I'm a lady, and at least moderately good at it.
Like others have said, long or short hair is a matter of taste. Would you get more girls interested in you if you cut it? Maybe, maybe not. But you would get a different kind of girl.
How we choose to look on the outside sends a signal to people about our personalities. If you wear nothing but tie-dyed shirts, then girls who don't like hippie-types aren't going to want to meet you. If you get a buzz cut and wear camo prints, then girls who don't like military types aren't going to want to meet you.
The good news is, you don't want to meet those particular girls either. When I see a guy with long hair, well taken care of, I generally think "A little out of the mainstream, maybe into music or art, or science fiction/fanatasy." That's because I most often see guys with long hair at events that attract artsy, sci-fi/fantasy, or gamer type people. Others might see long hair and go, "Oh, a hippie." Who knows?
The point being, be yourself and keep your hair.

It's an indicator to us girls whether or not we might have something in common with you.