Quest buy was my personal favourite off-earth setting, purely for how amusingly mundane it was treated. However I can understand that feeling of the worlds not feeling fleshed out enough. I personally didn't feel that way as I could appreciate the constraints of 11 minute episodes, but it did grate at times with places like St. O's that had been built up so highly.
I suppose that comes down to how well the world building was handled, I think Pixtopia did it well enough as does anything Mewnie related, but outside of that it's rather lacking.
That being said I have no qualms with the main focus being Star interacting with things on Earth, far from it as I actually prefer that theme. However, the way season 2 went about it felt rather disconnected from season 1. It reduced a lot of the big attractions of the first season's dynamics whilst trying some newer things that just didn't hit the mark. There were also a few sub-plots from season 1 which went nowhere or were just handled a bit hap-haphazardly, like the whole Marco romance and Miss Heinous' revenge.
The key thing in my mind is that, if the show wants the focus to be on Star's adventures with mundane life on earth it has to establish secondary and tertiary characters to provide fresh plot lines and dynamics without just making them the entire focus of an episode. I wish we'd learnt more about Jackie before the next episode, I wish we'd seen more from Tom in the wake of his supposed change of heart in Mr Candle Cares and other such grievances.