They seem to have weird resistance on it, but now with mod we can test it.
Here’s a list of all Attack that deal more damage on Headshot than on Bodyshot on Maulers (I had 0% damage against armor, infantry or Chaos for this test)
KRUBER :
-Executionner sword charged attack
-Halberd third light attack (Downstrike) and push attack
-1h Mace fourth light attack (Downstrike) and charged attack
-1h Sword third light attack (Downstrike)
-Sword & Shield third light attack (Lateral) and second charged attack (Pierce)
-All 2h Hammer attack EXCEPT charged attack
BARDIN :
-All 1h Axe attack
-All Axe & Shield attack
-All 2h Axe attack EXCEPT the push attack
-1h Hammer fourth light attack (Downstrike) and charged attack
-All 2h Hammer attack EXCEPT charged attack
-Pickaxe push attack
-Dual axes push and charged attack
KERILLIAN :
-Dual daggers charged attack
-Glaive first charged attack (Rising)
-1h Sword first two charged attack (Downstrike)
-Sword & Dagger second charged attack
SALTZPYRE :
-All 1h Axe attack
-Falchion third light attack (Downstrike)
-Flail two last light attack (Downstrike)
SIENNA :
-Dagger second charged attack (Pierce)
-Mace first light attack (Downstrike)
-Sword third light attack (Downstrike)
Note : All ranged weapon deals more Bodyshot damage than Headshot
-Fun fact : Light attack from swift bow deals more damage on headshot than charged attack (both still deal less than bodyshot)
-Fun fact bis : I was impossible to score and headshot with the first Sienna’s Dagger charged attack, so maybe it deals more headshot damage, but I couldn’t hit it x)
So yeah, to sum up, unless your weapon is an Axe, only Downstrike or Pierce attack deals more damage on headshot. And yet some of them doesn’t follow this rule (Glaive is an axe, and the second charged attack is a downstrike, and yet it deals less damage on headshot. Same goes with the Rapier charged attack, which is a Pierce)
EDIT : Maulers WERE harmed during these reasearches