The Dominion in the mirror universe is even more savage and ruthless than its counterpart in the 'Federation universe'.

For example, the Founders of the mirror universe never tolerated the defiance of the Teplan people by merely inflicting them with the "blight". (DS9 episode: "The Quickening") Instead, they ordered the entire mass extermination of the planet's population by using a different biogenic weapon. Within two weeks, the inhabitants had perished to the last man, and the planet was cracked open to be mined of all its mineral wealth, leaving behind a completely dead world. Such a brutal act was intended to be a reminder to all who opposed the will of the Dominion.

Even the servants of the Founders differ in the mirror universe. The Jem'Hadar are larger, stronger, and even more suicidally devoted to their masters. As for the Vorta, they are far more vicious, and possess more advanced telekinetic abilities to use in their role as either diplomats or assassins.

The mirror Dominion is also aggressive and expansionistic to a greater degree than its Federation universe counterpart. (Decipher RPG module: Through a Glass, Darkly)