I think Maria is attracted to Nathan. But her flirting with him, was really to do with Tyrone flirting with Fiz, or so she made it out to be. I'm not sure that she is completely happy with Tyrone, and I don't think it's fair for her to string him along if she isn't. I think Tyrone is great, but I think Maria feels that she is settling for second best.
I don't think it's fair to call Maria a slag, she's young and confused. It can be difficult to chose from a world that's safe, and one which is full of adventure. Both Tyrone and Nathan have different qualities, it's up to her what kind of life she choses.