Homer J, When an animated show is produced, the actors first record all the lines on tape and the animators draw the cels so that the mouths match the voices. Animation takes a long time to do so that particular episode was drawn a long time before it aired. So long, in fact, that they couldn’t have known who was going to be in the Superbowl at the time it was drawn. Therefore they just covered the mouths and dubbed in the correct team names. By hiding their mouths, they don’t have to worry about their mouths appearing to say the wrong team’s name.
Yes, the South Park boys have done some amazingly fast turn around times from drawing-to-airtime but that’s pretty rare.