Alot of the time, saying stuff like "our robot was the best, we should have won, ours should have been picked into finals, etc..." is pretty annoying. If your robot really was the best robot out there, wait for other teams to say something about it. If another team says your robot rocked, then it did. But when you see someone promoting how thier own robot did, it doesn't give much of a reason for anybody to listen because it's given that they loved thier robot.
If you need to show off how awesome your robot was, post a link to a video. That way, people could find something that might help them agree.
In all, I'd say it's ok to brag as long as you include other teams and dont say that yours was the best. That's for the other teams to decide. Self proclamed glory is worth nothing.