I went with slightly mature. She seemed to handle her father's death pretty well, and she never had any real money problems (though I don't seem to remember her having a job). She handled Joe pretty well, being very patient and tolerant of his little problem, but finally letting go when it was obviously hopeless. At one point she was going to leave Collinsport and go to Boston or someplace - I guess it takes some maturity to say goodbye to your lifelong home and venture out to someplace unknown. On the other hand, she was a bit immature in not seeing Nicolas Blair as the manipulative smooth operator he was. And she was almost always bubbly. That was my favorite thing about Maggie, always cheerful and ready to laugh. But I guess that's not exactly what one means by "mature". Of course, I'm not at all a fan of maturity, I usually relate it to people who are boring, humorless, and have a large stock portfolio.
So yeah, rather mature, but not overboard.