I'm not really seeing a problem here. That's how I was at that age. You said that she is an artist, so perhaps that's why she is the way she is. I'm an artist as well and have never stopped "playing". That's just how we roll. It's not really a Maturity issue, it's just how the brain of an artist works. We are more playful than others and more independant when it comes to social life.
As a teen I was always painting my room with "weird" things. I just love expressing myself differently. I am now 40 and my husband and I have a WALL-E bedspread! I am still as playful as I have ever been but also know when to get my hands dirty and get the important things done.
Let her be herself. She's an A student; smart and creative. Let her have her imagination. My parents never tried to change my "quirkiness" and I thank them everyday for it. By not "growing up" in the normal standards of society, I was able to freely expand my creativity and nail a job that I not only love, but supports a family of 4!
I also did not have many friends. But that's how I wanted it. I was very selective with my friends and the ones I had were loyal and good. I never got in trouble with them and still get together with them once and a while. Perhaps she is happy belonging to a small group.