Kecleon is a hidden Pokémon and does not appear in the game. It's likely that the developers decided not to include it for gameplay reasons.
It's worth noting that Kecleon is a somewhat unique Pokémon in the main series games, known for its ability to blend in with its surroundings and only becoming visible when attacked. It's possible that it wouldn't have translated well into the gameplay style of Pokémon Rumble Blast.