Pets have three moods: Neutral, Happy, and Sad. During the event, the pet's mood will worsen if you don't feed it regularly enough.
Once you fully level up a pet, it will appear on your battle screen, and its mood will change dynamically depending on your performance:

Happy: Shown when you defeat an enemy

Neutral: The pet’s normal state during battle

Sad: Appears when your dragon is defeated lose a battle