bandwagon fan

にわかファン
to jump on the bandwagon : 流行に飛びつく

Ex

• She always jumps on the bandwagon.
• I’m just a bandwagon fan.

Reference source

• https://www.gabastyle.com/english/naruhodo/naruhodo308/
• https://eikaiwa.dmm.com/uknow/questions/4413/