Your college team plays basketball. They’re good at basketball. Not the best, but good. Tonight you stand and cheer for your team in your collegiate arena. The team plays one of the actual best. A perennial contender as the cream of the crop.
But, tonight, your team, at home, gets the best of the usually flawless opponent. As the final seconds tick away it's clear that your team will emerge victorious! Suddenly a cheer, slowly and organically begins to build from the stands. The crowd mocks the opponents, chanting in unison, “OVERRATED. OVERRATED.”
As the time comes for you to lend your voice to the mass you stop and give question. 'What does an overrated opponent mean?' And, more importantly, because you don't really care about the opponent, 'what does that chant say about your school?'
Calling the opponents overrated implies they are not as good as predicted. As if actually losing weren't enough, now you're criticizing the perception of them! Burn. But, your team currently leads them. The scoreboard already signifies your team's superiority. Taking the focus off the opponents, what does the "OVERRATED" chant say about your own team? It says that the team the scoreboard currently indicates is better, your team, is beating a team not as good as portrayed? How could your group of schmucks possibly beat a team actually as good as others thought the opponents to be?
You love your team. You love your school. You want your team seen as the best! And your team appears better by surmounting the highest peak. The more formidable the opponent appears the better the victor appears.
So, devaluing the skill of the opponent ends up devaluing your own team. Instead, inflating the obviously defeated opponent inflates your obviously victorious own team. To me it makes more sense for the stadium to cheer, "UNDERRATED. UNDERRATED."
It most likely still accomplishes the original primary task of mocking the opponent. The opposing team probably isn't ignorant of the score (they are usually fairly good at basketball after all). Therefore chanting "UNDERRATED" juxtaposes the emotions of losing the game and a sarcastic implication from the crowd. Then it additionally implies that your team deserves to beat the opponent and would realize the same result against an even more highly regarded team.
In the end, I think that chanting, "OVERRATED" stems from a short sighted attempt to injure the opponent without considering the longterm implications. A much smarter, and to my way of thinking, more accurate approach, would actually result in chanting, "UNDERRATED" at the losers. A more sophisticated approach to the mob mentality.