Could it be just the natural lighting that make everything look less appealing to some?
Yes I think the overall brightness of the episode has caused two reactions in people. First of all SW doesn’t tend to look like that. Yes there’s been sunny settings like Cloud City, Endor & Naboo, but there’s clearly something about Rodriguez’s style, the colour grading or maybe simply the look of digital photography that made it stand out in this episode. Secondly, sunny exteriors are always to be avoided when relying on CG effects, especially when dealing with TV budgets. If you look closely, all of the things I mentioned above have been done in this series before, but it’s been done sparingly and quite often hidden in dark or grey lighting. The more hard directional lighting a scene have the better CG usually looks. The moment people try to add add even lighting it usually fails unless the budget is big enough to pull of a highly detailed CG character. It also has a lot to do with genre. Something like the Avengers movies have so much CG that people have been numbed to it, while SW still carries that old-school style to it. This show especially has deliberately used CG fairly sparingly opting to do things practically when possible, so all CG stands out so much more. And from even just the few Rodriguez films I’ve seen he clearly doesn’t shy away from CG.
It’s also kinda weird to me after seeing troopers looking so dirty that Moff Gideon has a nice fresh legion at his command.
Maybe the dirty ones where just Herzog’s Navarro stormtroopers and Gideon has much better resources? Either that, or Gideon and Thrawn’s other allies (I’m assuming they work together) has been regaining a lot of power over the course of the last two seasons.