Replaced two prong outlets with three prong and pigtailed ground to neutral. Bad idea?

Yes, very bad idea, not safe, and a Code violation because of the unsafe condition created when this practice is done.

To change two wire receptacles to 3 wire, you have several choices:

#1. Protect the entire receptacle circuit with a Ground Fault circuit breaker, and label each three wire receptacle with "No Equipment Ground"

#2. Protect all three wire receptacles with a Ground Fault receptacle and label each three wire receptacle with "No Equipment Ground", either with one GFI receptacle at the beginning of the circuit to protect all other standard receptacles downstream, or use a Ground Fault receptacle at each receptacle, and label each GFI receptacle with "No Equipment Ground".

#3. Install new wiring to each receptacle with a equipment grounding conductor.

There is a clause that allows a equipment grounding conductor be installed to each new three wire receptacle and connected to the system grounding electrode, however, if a equipment grounding conductor can be installed, then new wiring can be done, is my opinion.