Stackable passive debuffs such as shattering blows do not effect monsters.
I have the shattering blows passive maxed which says there is a 25% chance to apply 1 stack of weakened armor onto the opponent, which reduces the opponents defense by 5% with a maximum of 5 stacks.
I took a lower level weapon on a level 218 dual raider, and had no offensive buffs other than attack speed. I attacked a barbed snapper brute which took about 1.5 minutes to kill using melees. My damage initially was around 345-375 per hit, and after a minute or so the damage output was the same.
If the stacks were applied I would be dealing more damage overtime.