Ratings is never a problem with the Superbowl. He might not pull in ratings like Madonna or Beyonce, but the ratings during half-time won't fall hard just because Bruno is performing.
Anyways the amount of exposure and promo he'll get will benefit him TREMENDOUSLY.
Though, I do think it would be better if it was Bruno and someone else co-headlining but Bruno is more than capable or carrying a performance like this and he does have enough hit material to fill in an 11-13 minute time slot.