While Nintendo allowed user reviews for digital games purchased off the eShop on the Nintendo 3DS, on the Switch, the capability was missing. It’s… still missing on the Switch itself, but you can now leave reviews for digital titles you purchase off the eShop on Nintendo’s online game store. Presumably, these ratings will then be shown off on the actual eShop.
To rate a game, you need to use your Nintendo Account, and not only do you need to own the game in question, but you also need to have played at least two hours of it (which is a cool way of ensuring minimal review bombing occurs). You can rate a game, put in what kind of a player you are, and even drop some text if you want to go in depth with your thoughts on the game in question.
So, um- let’s hope people don’t troll or misuse this, because Nintendo usually cracks down on that by closing the services in question entirely (RIP SwapNote).