Oohohoho I remember that feeling.
Girl back in 6th form, I ended up head over heels with her but unfortunately she already had someone right through it. We'd talk about anything and I'd happily help with whatever I could, then again I'd help any of my friends if they needed it. It got to the point where we both admitted our feelings for eachother but we knew it could go no further. As much as I was infatuated by her on every level, I was ok with just being a close friend, because I could still spend time with her, be invited to birthday parties, I even got along well with her parents! Some may call this being in the Marinas Trench of the friendzone but I didn't care, at least I got to be comfortably close to someone whose company I enjoyed. I respectfully made my exit when she was with her boyfriend though.
So as you mentioned, she's taken, so there's no way of barging your way into a relationship with her. There's nothing wrong with being 'too nice', it's just your nature and it's most likely quite endearing to a lot of girls. Just make sure you don't act like she's the source of all light in the world, because you'll just be setting yourself up for a hard time.
But by all means feel free to continue spending time with her if you're comfortable how it is right now, don't feel bad about being too nice.