I play my Switch on a 65" TCL 2019 6-Series 4K TV and yes the games look stunning and incredibly sharp. However, every game is different and the majority of the titles I play are Nintendo-developed ones with their clean artstyle and (generally) native 1080p output. I think the only 3rd-party game I even own on it is Marvel Ultimate Alliance 3, which was still published by Nintendo actually haha, and is still 1080p.
I would assume some other titles, especially those that max out at 900p or lower, would tend to look worse or "blurry." The vast majority of 4K TV's have GREAT upscallers onboard for 1080p content, as that is still most TV/Blu-rays/game consoles and video streaming content. The issues happen when you go lower than 1080p, because that is considered more DVD-territory and a lesser area of focus for them, meaning worse image quality.
