Normally I would agree with the spiritual discipline of dressing up for Mass. I think receiving the Body, Blood, Soul and Divinity of Our Lord should be a notch or two above watching the Super Bowl Game on our priority list.
Having said that, though, I know a wonderful priest who makes a point of saying that he doesn't care one way or the other what you where to Mass in his parish. He doesn't mind if the altar servers wear sneakers or the parishoners wear jeans. This might be because the parish is in an economically depressed and somewhat dangerous part of the city. His parish is truly a refuge in the midst of the modern urban storm.