I think the earliest instance of "brain worms" I remember was from an episode of Invader Zim in 2001. Zim is offended at the concept of paying to ride a filthy, poorly maintained bus and shouts at the driver "Have you the brain worms?!"

Edit: found the video https://youtu.be/BiqTTozWNhA?si=bxNeTnreb77vdd00