noun
bandwagon fan; fair-weather fan
Refers to someone who suddenly becomes a fan of a team, artist, or trend only after it becomes popular or successful, often with a negative connotation of lacking genuine loyalty.
優勝した途端ににわかファンが増えた。
As soon as they won the championship, bandwagon fans increased.
彼はにわかファンだから、チームの歴史をよく知らない。
He's a fair-weather fan, so he doesn't know much about the team's history.