I usually use the Excel importer so that I can import an excel file that contains a lot of data, then you can quickly and easily put a lot of data in your app.
In addition to Sjors answer, if you don’t have much data, you could create a microflow to just create and commit individual pieces of data. I tend to use this approach for setting up things and call the microflow as part of the after startup microflow so this is setup automatically when the application is installed in an environment. When I do this, I also add a check to see if the data already exists before attempting to creating it.
And when you want to create mock users this API might be usefull: https://randomuser.me/