Ive been having this problem since at least 15w47. Mobs dont spawn very much at all in multiplayer. I couldnt find an open issue like this so i created one to make sure this is known before 1.9 drops.
MC-97186
Mob Spawning Greatly DECREASED in 1.9 multiplayer snapshots
Resolved
Awaiting Response
0
Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w42a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time.