Can iPad be used as a second display for the Mac mini?

iPad can be used as a second display for the Mac mini. Use an iPad as a second display for a Mac - Apple Support

I don't believe it can be the primary display. And, you'll need a primary display and a keyboard to configure the iPad for that use.

Apple makes Magic Keyboards for iPad and Mac, but I don't know that they are interchangeable. That is to say I don't know that a MK for iPad can be used with a Mac and vice versa.

If mobility is what you desire, the best route would be some flavor of MacBook - probably the MB Air. You could pick it up and take it with you when you want to be mobile, and you can connect it to a proper monitor when you're at your desk in your office or at home.

Though it is a very small form factor, the Mac mini isn't really designed to be a mobile computer. I doubt Apple wants to allow it to cannibalize MacBook sales by making it easy to connect the mini to portable accessories.

I understand your reasoning for wanting to use the iPad as a monitor for the mini, and I agree that the current implementation is less than ideal. But there's hope that things may change in the future.

If you're so inclined, you can be proactive and let Apple know that you'd like to see this feature developed by leaving direct feedback.

Product Feedback - Apple