Pokemon: Let's Go Pikachu/Eevee has a very clearly defined visual approach. It's simplistic yet pleasing, which is something the dev team was aiming for. They want the player to feel welcomed by the visuals, rather than bombarded by them. In an interview with IGN, lead game environment designer Kensaku Nabana discusses how the game almost took a very different approach to visuals.
“Usually, when you would have much more power with the Switch and taking it into HD, I think the natural tendency is to go for a more photorealistic approach. In the earliest days of development, we actually explored that direction quite a bit. But we got the direction from Masuda, just like he just mentioned a moment ago, to go for this more kind and soothing and inviting experience with the visuals. After some experimentation, we just realized the more realistic, more photorealistic direction just wasn't really working for what we were trying to do.”