Yes, not every game will let you use one without an adapter to mimic being a controller but some do. For example Fortnite on Switch will let you use a mouse and keyboard.

There are several games that support keyboard and mouse on Xbox

Here's the most complete list I found while researching the subject a few months ago:

https://xboxkbm.herokuapp.com/

For text entry, sure. For all games, not really - some game manufacturers would like competition to stay 'fair' and stick to controllers.