Hey, where would Cantopop be without covers?  It is a venerable tradition, to be sure.

I think the American song would have sounded better sung in Cantonese than in Mandarin, though.  The initial consonants are generally harder in Canto, like they are in English, and the vowels don't have that weird rounded+nasalized quality that can feel like French that you hear in Mando. (French rock, anyone?  I think you get my drift.)   Just a preference of mine, but generally I prefer pop music in Cantonese to Mandarin.  I like the sound of it better (not that I can understand one better than another, which is perhaps why I notice the actual sound of the words so much).  For pop music, maybe it does matter what language you sing in.  If the original song is in English, to reproduce the feeling of the song, you don't just need to take into account the styling of the original musical accompaniment, but also the sound of words of the original lyrics.  It does matter, at least to my ears.