Mmh... In my opinion, cello is too melancholic for the current Luna. I could picture her with a cello when she was on the moon, but now I like too see her cheerful.
Of course, you can play cheerful songs with a cello... But I've played cello for a while, and I don't think it's the best instrument for that. Although it's also possible that I wasn't good enough at it. x)