"Is it a bad idea to stand a light weight monitor on top of a mac mini?"
I wouldn't do this, no way.
Things are pretty tight and compact inside the Mini case, any weight on top would probably introduce some downward pressure that could damage internal connections and components.
Years and years ago, I had the very first PowerPC-based Mac, the PowerMac 6100. It had a low, narrow case, and I put an AppleColor monitor on top of it, which seemed to fit ok.
But after a while, I began to have all sorts of problems with the 6100. Pressing on the case would clear them up. I wasn't sure what was going on, and had the motherboard replaced, which was ok for a while, but then the problems returned. I surmised that the weight of the monitor (of course the AppleColor was a tube-based monitor, heavier than today's LCD's) was actually "flexing" the motherboard inside the 6100 case, and damaging it.
Lesson learned. No more "monitors on top of the case" for me.