I do understand why people are saying it is plain. There should be a lot more trees and actual forest areas. We haven't seen much and they have only really focused on the one area in the trailers we have gotten, so I don't think it is fair to judge the entire game by that only.
Hokkaido, the inspiration for SInnoh, does have many plains which is why everything is so flat and empty. This could be why it seems empty because plains are generally empty. They also have the largest marsh land in Japan, which is what the Saffari Zone like area is based on in Sinnoh.
Going by the in game map, the clouds could be blocking some stuff. I say this because they are blocking some of the main areas of the game, including areas in which we didn't go to in Pokemon DPPt and went to in Pokemon Ranger games.
So as I said, I really really don't think it is fair to judge the entire game on an area of the game which is inspired by plains. Not to mention, there is most likely DLC throughout the year of 2022.
We saw very little of Sinnoh in the actual games. I feel like this game will cover far more than what we know about.