There are a few (unofficial) moddable games if you look into the homebrew scene. But for this you need to have a Switch that is capable of running unofficial code (either old Switch with a softmod or a Switch with a hardmod)
If you have such a Switch you can use a tool that overwrites/unions the content of the game cartridge with content of a folder on your SD card. That means you could add some files (e.G. maps) into the game.
From the top of my head you can definitely mod the Skyrim Switch version as well as the Star Wars Jedi Knight Games (at least these were the two games which I've played around it). But I am pretty sure that there are Mods for the mario games.
Depends what you mean by "moddable". There are plenty of games that allow for user-generated content, like Super Mario Maker 2 or Baba Is You, for example. That might not be as moddable as a system that lets you run arbitrary code, but that seems more grey than black-and-white, to me.
If you have such a Switch you can use a tool that overwrites/unions the content of the game cartridge with content of a folder on your SD card. That means you could add some files (e.G. maps) into the game.
From the top of my head you can definitely mod the Skyrim Switch version as well as the Star Wars Jedi Knight Games (at least these were the two games which I've played around it). But I am pretty sure that there are Mods for the mario games.