Should activity monitor be using non-trivial CPU %? I noticed my Mac Mini was thrashing the fac despite not really running much, in the process of trying to figure out why I was surprised to see Activity Monitor itself bouncing around 8-12% CPU.

Is this expected?
Yes, it is expected. It has a lot to keep track of sometimes, and in the scheme of things, 8%-12% CPU is rather trivial. I've never seen it spike the CPU all by itself in years of looking at it.