A fairweather fan is someone who is only interested in a sports team when it is doing well. The term may also be used to describe political followers, music fans, and followers of various figures in the entertainment industry such as directors, actors, and so forth. As a general rule, fairweather fans are heavily criticized by people who regard themselves a loyal fans, sticking with a team through bad times as well as good ones.
This term is especially common in sports, thanks to the fact that the fortunes of a sports team are easy to follow and understand. A fairweather fan tends to root for the team which is doing well, ignoring that team if it starts to fail and sometimes switching loyalties, even to an opponent. People sometimes refer to fairweather fans as “bandwagoners,” referencing the idea that they “jump on the bandwagon” of a team which is doing well.
A number of things influence loyalty to sports teams and other entities in the entertainment world. Often, the entertainment value is a major factor. It isn't very much fun, for example, to watch a team lose over and over again, especially if it's losing to lesser teams or suffering from technicalities. The history of the team may also be important, with fans tending to be more loyal to teams with an old history. Attitudes of other fans are also important: Red Sox fans, for example, have bonded over the remarkably unlucky history of their favorite team, creating a sort of group solidarity.
Unlike fairweather fans, loyal fans stick with a team even during long dry spells between championships, such as what the Red Sox endured in baseball.
People who consider themselves “true” fans often sneer at a fairweather fan. Some fairweather fans go overboard with team support, decking themselves out in branded apparel, hanging team flags in their homes, and so forth, and this outward display is sometimes viewed as a cause for mockery. The very term “fairweather fan” is a bit pejorative, as it calls up images of the “fairweather friend” who only seems to be around when he or she can benefit.
Like a regular fan, a fairweather fan can be any age, with a varying level of interest in the sport. Someone who doesn't follow a sport that closely, for example, might be a fairweather fan simply because he or she doesn't feel enough of a connection with the sport to be invested in the outcome of a particular team. Conversely, someone who follows a sport very closely might support winning teams because they go further, providing more opportunities to watch or attend games. People may also become fairweather fans to share an interest with friends and loved ones, attaching their loyalties to those of their friends.
https://www.sportsnhobbies.org/what-is-a-fairweather-fan.htm