So I've been told by both FamousAmos and Tolvo Dusk Crown doesn't affect miracles, I didn't believe them so its time to test!

Dark Souls Remastered What Increases Miracles Dmg Full

I'm on my SL120 Dex/Fth build testing on the hollows in the duel area, I'm on NG. My character has 45 Fth, and with a Darkmoon Talisman 208 Magic Adjust. I'll test a few spells.
WoG:

Sep 09, 2013  Surprisingly the only miracle to get a benefit from dex is lightning spear. I suggest avoiding dex on a miracle caster build unless you plan on using all three lightning spear variants.:P.

Without Crown:
Hollow Warrior 525 Dmg
Crossbow Soldier: 525 Dmg
Spear Soldier 524 Dmg/209 Shield Up
With Crown:
Hollow Warrior: 630 Dmg
Crossbow Soldier: 630 Dmg
Spear Soldier: 639 Dmg/252 Dmg
Great Lightning Spear:
Without Crown:
All Same 562 Dmg and 252 through shield
With Crown
All Same: 675 Dmg and 275 through shield
Heal

Dark Souls Remastered Pc

Without Crown
Healed from 206 to 831, so 625 HP

Dark Souls Remastered What Increases Miracles Dmg 2017

With Crown
Healed from 175 to 800, so 625 HP
So Dusk Crown buffs offensive miracles, but not heals. Now you know