Sitting in your car with the AC on is bad for extended periods as prolonged idling is bad for the engine.
Prolonged engine idling especially with the AC on can potentially result in engine overheating, increased engine wear and poor fuel efficiency.
The AC compressor that makes your AC work in your car also puts additional strain on the engine, which also can increase the chances of the engine overheating.
Although for short periods sitting in the car with the AC on is not bad, but you shouldn't sit in the car for hours with the AC on but for 30 minutes or less it's usually okay.
Also never sit in the car with the engine running or engine and Ac running in an enclosed space like a parking garage, garage, etc as it can lead to the buildup of deadly carbon monoxide.
A vehicles engine also needs air movement such as from driving down the road to help keep the engine cool, along with the engine fan.
The radiator needs airflow from driving to also cool the radiator.
And extended engine idling also places extra, unnecessary strain on the engine components and results in excessive wear over time.
And idling the engine also uses more fuel without moving the vehicle and the fuel consumption increases with the AC on.
Also be mindful of people and your surroundings even when outdoors, as idling the car with the AC on or idling the car in general in a parking lot or nearby others can affect those people with noise and fumes.
If you can, try parking the vehicle in the shade and turn off the engine and then restart it when it gets hot again.
If you must idle the engine with the AC on you should keep an eye on the temperature gauge and if the engine starts to overheat you should shut it off and allow it to cool down.
Overheating of the engine can result in a damaged engine, warped engine head and failed head gasket.