I think it would be just as good if it was another character, however it wouldn't be given as much of a chance to people who haven't yet played it... however, i also think a major factor in Mario Galaxy (one of my favourite aspects of what we've seen so far) is the nostalgia from previous mario games
Oh, and out of the Mario Kart games i've played (all except SNES), Mario Kart 64 is by far my favourite, i've spent so much time on it, and still play it over the other two when me and my friends/cousins feel like playing