What causes the increased power consumption of a window air conditioner in the afternoon?
The air conditioner has to work harder when it has to work against a higher temperature difference.  This changes the relative pressures inside the cooling unit, which puts more back pressure on the compressor, which puts more load on the motor, which draws more power.

The temperature coefficient of the wire resistance is a tiny irrelevant fraction of the difference.  Copper does change resistivity a tiny amount over the temperature range you mention, but in either case the copper wires are only responsible for a very small fraction of the overall electric power going into the air conditioner.