I've had the impression that in the Mirror Universe, it was only the humans/terrans which behaved differently. Everyone else we see seems like their usual selves (accounting for different circumstances of course).
The DS9 episodes throw a wrench in this of course, with Kira being a very different person and the oppressed terrans being sympathetic.
In regard to the Breen, I would note that Star Trek Adventures: Klingon Core Rulebook says "They only
present themselves to non-Breen in full-body suits that hide
their identities, and their bodies disintegrate when they die
while wearing those suits. While this is an admirable trait – it
means Breen cannot be taken prisoner, an attitude in line
with the teachings of Kahless – it has made it impossible to
determine the true face of the Breen." This is a pretty cool explanation for how Worf could be right even after Kira and Dukat stole their suits.
I know STA isn't canon but I still like this reasoning.
This is the second Star Trek season premiere that features the destruction of a Soong-type android, after Picard S1E1 "Rememberance" introduced and murdered Dahj.