I think in a way it's main exclusive games are not only more immature or childish, but also seem to have an extremely obsessed cult (of adults) that worship them. It's not particularly a trend specific to Nintendo. Sony, for example, has their Naughty Dog / The Last of Us cult that worships the ground they walk on. It's just that when you see a lot of soy boys, Mario, Zelda, Harry Potter (not Switch I know but I think an example you see outside of gaming), Pokemon, are part of these people's identities. Add in also the fact that Animal Crossing turned into not a kids game but basically a place for leftists to make their pretend utopia. It's just that the Switch library is where these people's obsessions gravitate.

To be clear, I think it's important for every adult to keep a bit of their inner child alive and not be afraid of enjoying games, movies, whatever even if it's more intended for kids. Hell, I might could even argue that some of it would be better to enjoy than the shitshow a lot of "adult" media has become. Not everything an adult does has to revolve around being a serious adult that only drinks black coffee and straight bourbon. The key difference is enjoying a fine piece of fictional entertainment versus making these characters, factions, whatever a key part of your identity.

I'm not ashamed at all to admit I've enjoyed building things in Minecraft or playing a Zelda game, but that's just something I do at home for fun. Everyone needs low effort downtime in their life and I think most adults fill that time with TV. I watch very little TV and choose instead video games and books of whatever type I find fun. The rest of the time when I'm doing adult things I don't wear my creeper hat, triforce t-shirt, and carry a diamond sword keychain. Because I'm not my 5 year old nephew.