Rabulias
the Incomparably Shrewd and Clever
Curiously, the item's description gives its AC and hit points, but doesn't describe the result of its destruction. Are the creatures within it all freed, or are they lost forever in their extraplanar spaces?
Given it's a mirror, I'd probably go with "smash the mirror, release those trapped within," but you're right - there's no explicit right answer. To each DM their own, then,
5th Edition DMG p.182 said:If the mirror is shattered, all creatures it contains are freed and appear in unoccupied spaces near it.