Well a fast way to import dummy data is bij importing through excel with the excel importer: https://appstore.home.mendix.com/link/app/72/
Another way to do it, is to create a microflow that creates the objects with dummy data
You can create a microflow to populate your application.
If you have a database, you can setup the mendix to use it. Go to project settings → Configurations → edit or create a new configuration. Then, you can select a database, and use it to your application.
And as Maarten said, you can use Excel importer.