If you could support the bottom of the Mini around the metal periphery of the bottom vice the plastic disk it would well support a 22" monitor.
The problem with being supported by the plastic disk with a heavy weight is that if the weight is not evenly distributed, the plastic may cold flow unevenly over time resulting in unequal pressure on the bottom and tilting of what is on top and even it falling off but I doubt it would go that far that fast. The Mini/plastic getting hot would accelerate the cold flow.
The metal part of the 2012 Mini (the one I have) should easily support even a Thunderbolt monitor.
Another problem with putting something on top is that it may attenuate wifi and BT reception.