A champion who holds an official title but lacks the caliber of a real champion.
Those boxers are just paper champions. Why? True, they may have real world titles. But they don't fight outside their country and they don't take on fighters other than their own nationality much.
A player and/or team that is statistically superior to one's opponent and is viewed as the almost guaranteed victor in an upcoming match, yet is unable to achieve victory over said opponent.
The undefeated New England Patriots were beaten by the New York Giants in Super Bowl XLII, proving the Patriots to be nothing but paper champs.