I always believed that the passenger cabin lights are dimmed so that your iris is open wider, so that in the event of incident, with total loss of light, evacuation would be those few seconds quicker! Less groping around getting in peoples way when seconds count.
Cockpit lighting is dimmed for the same reason, this is why most switches are illuminated and less iris adjustment is required from the pilots when throwing focus between inside and outside the cockpit at night.
"Light pollution" it often gets called. I thought of that one, and assumed that the wings (the only reflective surface visible from inside the aircraft) were so far aft of the cockpit that it wouldn't be a problem, so discounted it, but then i thought about reflection off cloud. It might be that that is more significant.