I disagree on this point. DS9 should not have done it at all, or at least not linked their alternate universe version to the TOS MU. Because their version was a complete bastardization of what was shown in "Mirror Mirror."
I enjoyed it as a one-off episode. Everyone gave well considered performances and the concept wasn't run into the ground. After this, it felt like every time they added a new semi-regular, they had to go a Mirror Universe episode with them included. Revisiting it one time was refreshing. Making it a yearly staple was what I didn't need.