The vast majority of it is absolutely atrocious - actors wildly flailing their arms with obviously zero understanding of what a conductor does and even less training in the basic patterns. And it's not just the actors. Editors need to understand how this works too, cause quite often they put downbeats in the wrong place and bounce around a ton, slipping the "conductor's" motion around shot to shot.
It's seriously one of the worst and laziest things you'll see all the time in movies about music.
I'd say it's usually even worse than actors "playing" instruments they have no idea how to play.