Well, I think that Dark Land is in perpetual Darkness, as has been the trend with wherever Bowzer dwells. We saw in Super Mario World that the area around his castle was always dark with an occasional bolt of lightning. Also, all the levels in Dark Land had dark skies, giving the impression of perpetual darkness.
Then again, if the background gave us any idea of the amount of sun/moon light, worlds 1-7 would be arid wastelands and world 8 a frozen tundra.
By the way, I'm new :?)