Is it okay to place my monitor on top of my mac mini?

Maybe a small monitor.

For one of my Mac Mini's, I used a piece of medium gauge sheet metal to make small shelf. It is 2 inches high x 10 inches wide x 6" deep and painted black. The Mac Mini fits inside and the monitor sits on top.

A small monitor is fine. I would say 24" or less modern monitor. I sure would not place a 24" CRT on the Mini

There are two issues that you need to consider, firstly as discussed by others here the weight of the monitor and again as others have mentioned an LCD monitor of up to 24 inches should not be a problem, and secondly heat issues. Again an LCD monitor with the usual type of stand these monitors have should not cause any heat problems nor should it cause any ventilation problems.

On the other hand a CRT monitor would be much, much heavier and would also likely put out more heat, and as many CRT monitors have no stand that lifts them up off what ever they are on top of would also be a potential heat problem as well.