I don't own any sports-team jerseys of my own, but the way I see it (I could be wrong), the owners of such jerseys usually fall into three camps:
camp A (least likely): the wearer is not actually a fan, but likes the look of the jersey, or likes to pretend to be "into" sports to appear cool or multidimensional
camp B (often): the wearer is some frustrated sports fan loser (usually fat, slow, and physically inferior) who has never accomplished anything on their own and would prefer to live through the accomplishments of others. This same person usually likes to accuse other people of homosexuality, or criticize the athleticism of others, while their own athletic endeavors are limited to dorito-eating.
camp C (most likely): the wearer simply admires the accomplishments of the athlete whose name appears on the jersey, and would like to see said athlete receive the attention and exposure they deserve. The wearer is also most likely a current or former athlete who bothers to stay competitive and in shape.
In other words, jersey owners are sometimes poseurs, often losers, but most likely just regular schmoes.