Yes.  There are many overweight singers who have made it big (no pun intended) without even having a killer band behind them. An interesting question for me is:  Do you think the band would be as successful if they were doing everything exactly the way they are now, but Grace had a mediocre voice?  Personally, I doubt there are that many fans coming primarily to see a sexy Grace; though it undoubtedly is an additional perk for those who appreciate it.