wouldn't do diddly-squat in terms of protection vs a copper hammer
It would prevent the full dirrect force of the hammer from impacting you thanks to the inertia of Chainmail including it can spread out the force of impact along its links.
The spreading out of the force would hardly even exist. They're just links woven together; pretty much all the force would be transferred. If it were a solid piece of metal, sure, but it's not; it's chain links being pressed directly against what's underneath it. No reason why the force would spread around much.
Chain is also going to be relatively light, so it wouldn't absorb much of the force anyway.
This is, as has been said, why padding is worn under metallic armor.
Even with solid piece of metal, the concussion shock from the metal plate transferring the power to you could easily injure you. Therefore hammer-strike will still cause damage to plate armor wearing individuals.
Armor - Resistive (level of resistivity)
Plate Armor - Slash (Med-High), Gore (High), Pierce (Med), Hammer (Low)
Chainmail - Slash(Med), Gore (Med-High), Pierce (High), Hammer (Low)
Leather - Vulnerable to most except Gore
Of course, plate armor reduces the individual's ability to avoid hits (they have to simply withstand the hit with their armor). And hammer should provide serious penalty to the dwarf ability to hit (slower), which compensate for hammer's ability to damage all armor type.
Which mean inherently, the heavier the armor, the more vulnerable a soldier is to high impact weapons. And high impact weapon have difficulty hit light enemy.
Another interesting thought, given that damage is inflicted by local acceleration to your body. Which means that with heavy armor, you don't move much when hit, which means all the impact is absorbed by the place the weapon hit you. However, if you're light enough, you can let your whole body move with impact, and reduce the relative acceleration to you local point.