I'd say "sort of." I see flat UI designs in games in essentially two places.
The first is games where it seems stylistically appropriate within the aesthetic of the game. I'd argue Pokemon falls into this category somewhat, but that's definitely eye of the beholder. The flat UIs feel congruent enough with the world to me.
The second place you'll see flatter UIs sometimes is in the sort of "hub" UIs that many multiplayer games. They usually have a lot of functionality, are very much "out of game," and are incidentally normally built on web tech.
But for regular, in-game UIs? Not really IMO, not like it has been for apps.
Yeah it's definitely eye of the beholder. A congruent example I can think of is a lot of immersive 3D games going more minimal on the UI, using plain text overlay and no visible style, e.g. Ghost of Tsushima, where I think the UI style helped the goal of "immersive". I can see Pokemon S/S is going for a more realistic or immersive 3D world and can benefit from a more minimal UI, but to me visually it turns out to resemble too much to the typical "flat design", and my personal visual taste is stopping me from understanding what it achieved on the game design level.
And I agree there're places like "hub" where it's more disconnected to the game world, and you can say it's less important for those to use a very consistent style. I would argue these places should also be fun and game-like (or, the UI interaction feedback should match with some of the in-game feedbacks), but that's more experimental and a different subject.
The first is games where it seems stylistically appropriate within the aesthetic of the game. I'd argue Pokemon falls into this category somewhat, but that's definitely eye of the beholder. The flat UIs feel congruent enough with the world to me.
The second place you'll see flatter UIs sometimes is in the sort of "hub" UIs that many multiplayer games. They usually have a lot of functionality, are very much "out of game," and are incidentally normally built on web tech.
But for regular, in-game UIs? Not really IMO, not like it has been for apps.