Aircraft are cooled or heated by the aircraft air conditioning packs. These need air, which comes from either the APU or engines. They also provide the air for pressurisation when in flight (787s get their air from electrically powered packs, so only need ground power but they are an exception). On the ground with engines off and APU off the only source of heat or cooling comes from ground air. This is provided (usually) by buit in conditioning units which you can sometimes see as big yellow hoses plugged into the undersides of the aircraft.

Overnight, unless the airport is used to and prepared for extreme weather aircraft are normally depowered overnight. It s not unusual (particularly on ShortHaul) for the crew to be the first people to a cold dark aircraft. If no air conditioning air is available then the crew would have to start the APU to provide heating. This is often restricted, but even if it is not the crew would not be at the aircraft much before the passengers. It takes a finite amount of time to heat or cool the aircraft.

Ground conditioning is, in many places only sufficient to provide temperature maintenance, heating or cooling is a slow process indeed. The packs are far far far better for this process. The mist is merely water vapour cold air coming from the vents hitting warm moist cabin air and leading to condensation. It is not indicative of efficient cooling. Only high mass flow of colder air can do this. Aircraft systems cool themselves far better than any ground equipment.