Bandwagoner

[Vania]

A person who starts to support a particular thing (sports, fashion, people, ect) simply because they are [trendy] or start [gaining] [momentum].

[Greta]

A [fake fan]. Wannabes who want to fit in with [whatevers] considered popular or hyped in order to gain attention and acceptance. They like something based on the [majority of peoples] interests, not their own. Once that sports team or trend is no longer popular, they will follow whatever is currently considered cool or popular.