A Mankey stole my looplet. Can I get it back?

There's no long worded answer for this. Simply no you can't. They're were boss NPCs with a unique id. You'll never find the same ones and every time you re enter the dungeon the game generates new NPCs with unique items(if they have any).

Items in Pokemon Mystery Dungeon aren't really rare, it shouldn't take long trying to get the same item back.