I agree with you, I also think she is a Pisces or Cancer rising. Either of these would explain her dreamy image and would emphasize (with aspect to moon in Cancer) her music that often center around the idealistic and romantic side of love. She comes off as very cute and sweet. She also is very open about her relationships and isn't afraid to express her feelings (and reveal who it's about) in her songs. This makes sense as her Sun is in Sagittarius. Actually now, I'm really leaning towards Pisces more than Cancer!
Well I would see her as a virgo rising. She looks young and pretty unassuming. Also, virgo risings have pisces on the descendant, which means that in love, they have a dreamy outlook, and her moon being in cancer, would probably be in 11th house which would make her have a need for friendship in emotional attachments which manifests in her song, you belong with me. In the song it seems that she's close to him but overlooked because she's more of a friend but its tat reason that she thinks they'd be good together in a romantic way.