Yes, there are design issues with high side switches, especially with MOSFETS. I prefer bipolar transistors for this purpose and have used the following circuit with a 3 volt PIR sensor. It has the same logic as the OP's original design (a high from the PIR turns on the pro mini). I added a diode to drop the 6V battery to < 5.5 which also provides reverse polarity protection. And now, everything has a common ground.
This circuit presents a very light load to the PIR (the one I used runs off of 2xAAA batteries for a couple of years and can't provide much drive).
The resistor values chosen will allow the pro-mini to draw over 100 mA, but of course it hasn't been tested with one. R2 could probably be increased to 5K or 10K (and R1 to 1M) if the pro mini draws much less.
