We often use messy to describe a situation before we tidy up. We might describe the playroom as messy, with a smile before we start tidying. Or we might use messy when we see our baby with food all over their face, before we start cleaning them up. Messy is just an observation (and not a criticism) that marks the transition to clean-up time.
Start by teaching messy based on real-life experiences, then use the flash card to help your baby learn to generalize the sign.