Is there a way to make my iPad act as a display for my Mac Mini

Install a VNC app on the iPad. Activate Screen Sharing on the Mac. Connect to the Mac from the iPad. It won't work very well. If you're thinking the iPad can replace a real display, forget it.

There are several VNC clients available for the iPad in the iTunes App Store.

Make your choice and install to the iPad. I chose Remoter VNC.

Now, the steps:

1. System Preferences > Sharing > Screen Sharing.
   1. In Computer Settings, check VNC viewers may control screen ... and OK.
   2. Set a password that the iPad VNC app will request.
2. You are alerted that Screen Sharing: is blocked by Firewall.

   Switch to Security & Privacy preference

   1. Click and unlock with your login password
   2. Leave Firewall running
   3. Choose Firewall options
   4. Deselect Block all incoming connections if it is checked
      1. You may now see an entry for Screen Sharing and Allow incoming connections
      2. Do not recheck Block all incoming connections
      3. Click OK
   5. Click to lock the firewall
3. On your iPad, launch the VNC application.
   1. In the VNC app, set the name for the VNC client.
   2. Configure your user id and password (from above).
   3. Configure other VNC app preferences including port 5900.
4. Launch your iPad VNC client and click to login.
5. Your OS X ML desktop and open windows will appear on the iPad.

And here is this current entry in the Apple Support Community in a VNC session on my iPad. It has been trimmed down to fit the paste here.

Surly the VNC is going to be quite slow though? Even with a good internet connection I find it can lag...

My example was on an original iPad connected via 5Ghz 802.11n. The Mac mini was delivering Gbit to the Airport Extreme, which was five feet distant from the iPad.

Your perception of slow is accurate. It would be painful with less network bandwidth, and more usably practical with Gen3 and Gen4 iPad performance improvements.

And, I expect with different supported features across VNC clients, some will be faster than others.

Succinct assessment.