Would it be totally bandwagon if....?

In my opinion you would be considered a bandwagon. The reason is because you don't know much about the sport nor the teams and just because the Celtics won, you chose to buy the clothing. If they hadn't won, you probably wouldn't. You are just trying to do what you see others doing. If I were to know you, and if you happened to be wearing that jersey, I would probably converse with you about the Celtics. Then finding out you didn't know much about them, then I would believe that you are a bandwagon.