Before Version 3.13, Mousse's and Mio's voice packs were the only voice packs in the game that didn't require DLC to unlock.
Mousse is the only monster with associated emotes.
While Mousse is not a playable character, Poppo can obtain the Mousse Poppo Costume, making her look identical to Mousse.
Mousse counts as a variant of Poppo for the purposes of activating character-specific voice lines.
The game contains assets for Mousse colors, which look identical to the colors used for the Mousse Poppo Costume. However, only the Orange, Green, and Chocolate Mousse colors have been used in-game.