Testing and debugging MongoDB Stitch functions is simplified by the MongoDB Stitch UI, which provides a simple way to display debug information from functions. Developers can test their functions through the UI by selecting a user for the function to run as and calling exports() with parameters. The results of the function call appear in the Results panel, while any console.log() output appears in the Logs panel. Additionally, the Stitch UI provides access to logs for production apps, allowing developers to monitor what's happening in their applications.