In Mirror Universe:Glass Empires the book concludes with a storyline that cliffhangs in the Klingon-Cardassian alliance fighting a Borg invasion-and suffering worse than Starfleet did .
No, MU Picard destroyed that Borg ship.
It was implied
the Borg were probably going to try again, but the story took place in 2371, and as of the last DS9 MU episode (in 2375?), the Borg didn't appear to an issue in the Mirror Universe.