Star Trek: Discovery definitely introduced an intriguing bit of lore to the Mirror Universe with this revelation, and hopefully future stories in the Star Trek franchise will further explore the rift between Mirror Sarek and Mirror Spock.
Both sons disobeyed their fathers’ idealized wishes, although it’s safe to say Mirror Spock’s betrayal is objectively much worse.
While the original Spock’s decision to join Starfleet was a bit rough, since Sarek was forced to choose him for the Vulcan Science Academy over his foster daughter Burnham, it’s still ten times better than Mirror Sarek losing his son to such a militantly fascist movement.