Yes prolonged diesel engine idling is bad for the diesel engine because the diesel engines combustion chamber temperatures can drop so low that the diesel fuel may not burn completely.
Idling the diesel engine too long also puts more wear and tear on the engine and shortens the engines lifespan.
The diesel engine can also overheat when it's just sitting there idling without air moving over the engine through the engines radiator.
This is especially true during hot temperatures such as in the summer heat as the diesel engines including gasoline engines can overheat.
During winter months running a diesel engine at idle for prolonged periods of time is not as bad because the cold air helps to cool the engine.
And sometimes diesel engines such as on semi trucks need to idle as much as possible to keep the diesel fuel from gelling in extreme cold and because it can sometimes be hard to get the diesel engine restarted in really cold weather.