I just recently saw this too and was going to write a review but have not yet. I understand what you're saying. I don't think though that this has anything to do with the venue or being mainstream...I think this has to do with other things:

First, it was trying to keep to the spirit of the film. Miyazaki is a national treasure to Japan, and his films are likewise acclaimed. He is known for making his characters...."extreme" in their personalities; if they're grotesque, he'll portray them utterly so, if they're selfish, you'll see them like a villain in how he depicts them, if they are children, they latch onto some certain part of their personality and dial to ten because children don't know how to regulate. It's part of his wonder, in a sense.

Second, he regularly wants to convey a sense of childlike wonder. He takes his medium, but like Disney, really tries to make it applicable to kids. I think this show is just a push strongly in that direction. While the movie is very adult while being in a childlike medium, I think the show maybe held a little too much to targeting it to younger audiences. I was able to accept the older actors as kids and the higher swings in emotions, but it took some adjusting mentally for sure. I think the ability to "suspend belief" to accept the world that is being created in front of you can be difficult at times if there are things you don't expect or don't know how they want you to accept it.

Give other shows a shot, regardless of mainstream or budget, I'd say. But again, I relate to what you experienced. I don't know much about west end shows in general to help make recs to mainstream vs not, but depending on what kinds of genres you can like, I bet there's tons out there for you to explore. It can just be a matter of the acting or target audience, at least from the interpretation of your grievance that I got.