I have a family room with 15 amp breaker feeding a few receptacles. I replaced an old receptacle but had to use 12 ga wire as pigtails on that one receptacle. Is that ok or no?
The 12ga wire is perfectly fine to use as a pigtail for a 15amp circuit.
Just note, The pigtail needs to be securely bound to the existing wiring. So a couple of twisted wires and electric tape is not enough, wire caps or something similar needs to be used. And if the existing wiring is aluminum, you will need to use specialized caps to bind copper wire to it.
As mentioned by diceless, that's perfectly acceptable, as long as it is connected properly, which was also mentioned.
Just wanted to note that the inverse is not acceptable, to use a 14ga jumper in a 20amp circuit. This runs the risk of that section of wire melting before the circuit breaker trips.
Where your 12ga jumper is able to handle more current than the rest of the 15amp circuit, you're all set.