Is it possible to display the list of categories / courses that we offer before registration for the course on the Mobile application?
the list of available courses and categories can only be accessed through the Site Home in the app. If your Site Home is configured to display Available courses, the app will display that option too (and the same for categories).
You can force the app to display Site Home instead of Dashboard when the app is started or the user logs in. This is done with the defaulthomepage setting in Moodle.