Two Mirri quesions

Petoften

What all chests does her bonus apply to? For example, if I open a chest from a treasure map? Or a chest spawned by killing a boss?

Does the quality of the chest affect the quality of her bonus loot? Would an intermediate chest have better bonus loot than a simple chest?

DoofusMax

It's just overland and Treasure Chests from maps, AFAIK. Other chests draw from different loot pools, so the Dragonguard Supply Chest or the boss chest at the end of Atoll of Immolation have different stuff and wouldn't drop a Hidden Bag from Mirri even though they are chests.

Mirri's Hidden Loot bags are not quality-dependent, just like Ember's wallets aren't affected by pickpocket passives like Cutpurse's Art. You either hit the chance to have one drop with the other contents or you don't and what's in the bag is its own loot pool (usually a small bit of extra gold).