Bart's a bit difficult - as I said he's only on screen less than a second. After that he disappears behind the crowd and where he sits on the couch is blocked. The best snap I can get of it is attached below (enlarge it). The one thing I can definitely tell just on regular speed is that the team logo has a vertical white line in the middle of it.
The shirt uses dark colors, top word seems to be quite longer than the bottom one, and there appears to be another white line going to horizontally going to the left on the left half of it.
Since it lines up right against Comic Book Guy's shirt logo, which turns out to help for comparison purposes, I'm really thinking that it's likely a Titans logo (that's the only NFL team logo I can find with a vertical white line in the middle and a horizontal white line)
So Bart Simpson's in a Titans shirt