After some Google (because it isn't documented on the hMod wiki) I've found that theres a parameter in hMod called natural-spawn-rate that is not put into your server.properties by default.
In your server.properties file you would use it like this:
now the value is a percentage chance for the mob to spawn, but it is backwards, so 2 = a 98% chance to spawn a mob/animal on a block.
means only a 25% chance for a mob to spawn.
This will hopefully take some of the load of your server whilst still allowing monster mobs to spawn at night (just not as many).
I haven't confirmed it works because I can't login to my server from work at the moment but I'll update once I know for sure it works.