My general dislike for that genre of music aside, I felt that the lyrics were totally irrelevant to the film. The song is about a teen girl reminiscing about her fling and the idyllic summer that they spent together. This really didn't seem appropriate for Arrietty, given the serious nature of the film. Both of the main characters were in life or death situations, and the bond that they eventually form is something much more deep and meaningful. It doesn't seem like the song was even written with this particular film in mind, they just decided to insert it, because it was a close enough fit.
Eh... I guess it really isn't a big deal in the grand scheme of things, but its inclusion in the film bothered me for some reason.
