This isn't a terrible movie, but is lacking in originality. I think that, given time, director Annie Baker will find her own voice instead of trying to be Greta Gerwig. If she works with Zoe Ziegler again, she'll hopefully stop trying to turn her into the new young Saoirse Ronan, too, and let Zoe act in her own way, as I suspect she can.
The basic plot here has been done, in various ways, in many, many far better movies. Baker tried to stuff too much in here, too. The subplot, if you can call it that, about the mother was incomplete and distracted from what the young girl was going through as she was dealing with the changes she was experiencing. Sure, the relation between the mother and daughter was integral to the film, but seemed incomplete, never resolved. It was never really examined in the depth it could have been. That might have been due to the slow pace of the movie. Too little happened over too much time.
The men in the movie were never really fleshed out, either. Most obvious was "Wayne", who was sort of a good guy, but at the same time, not. We never really got to know anything about him, other than he had a nice daughter.
By sheer coincidence I just saw "Are You There God? It's Me, Margaret." a couple of days before I saw "Janet Planet". One could argue they are wildly different movies, but at the same time, they do cover similar topics. "Are You There God? It's Me, Margaret." is far better, more entertaining, and more realistic. It's fun. "Janet Planet" is dull in comparison.
But the biggest and most obvious comparison that jumps out at you as you watch "Janet Planet" would be between it and "Lady Bird". They are much more similar films than the one I mentioned above. And "Lady Bird" is much, much better.
As I said, this isn't a terrible film. I look forward to seeing what Baker can do in the future. I'm sure she has a much better film in her.
As an aside, the best song in the movie was "Miracle Man" by Bob Carpenter. Too bad it was associated with the "Wayne" character.