Place microwave popcorn pouch in the microwave with folds facing up. Microwave on high until popping slows to 1 to 2 seconds between pops, 1 1/2 to 2 1/2 minutes. Carefully open bag by pulling diagonally at corners and pour popcorn into a large bowl.
Melt white chocolate in a microwave-safe glass or ceramic bowl in 15-second intervals, stirring after each melting, 1 to 3 minutes.
Stir melted white chocolate and crushed peppermint sweets into popcorn. Spread evenly on a baking tray; allow to cool and harden, about 15 minutes.