Indeed it isn't. Even on Forsaken where we removed ores from the ceiling and increased mob danger on the surface significantly, underground it feels.... Almost entirely safe. Which since you're there to basically do nothing but mine.... If only we had evil worms to burrow through the ground.
EDIT: Fixed my fails.
Yes, last I remember mobs were supposed to spawn regardless of light level in the DD. This may no longer be the case it seems.