via Wikipedia: Recent observations
While the first five Super Bowl winners of the 2000s posted above average winning percentages the year following their Super Bowl appearance, the losers of the same games posted below average winning percentages in the follow-up year.[3] The Super Bowl curse is also said to apply to winners of the game, as since 1993 few winning teams have followed up their Super Bowl successes with a second Super Bowl appearance (Denver Broncos, Dallas Cowboys and New England Patriots won; Green Bay Packers lost) or even advanced to a conference title game in the subsequent season (Dallas Cowboys).