Mewtwo is not a part of the Pokémon Diamond game's regular Pokédex. It is not obtainable in the game through normal gameplay, even with cheats or glitches.
Mewtwo is only available in specific Pokémon games:
* Pokémon Red, Blue, and Yellow: As a legendary Pokémon, Mewtwo can be found in the Cerulean Cave after defeating the Elite Four.
* Pokémon Let's Go Pikachu and Let's Go Eevee: Mewtwo can be encountered after completing the game and fulfilling certain requirements.
* Pokémon Go: Mewtwo is a legendary Pokémon that can be encountered in raids.
* Pokémon Sword and Shield: Mewtwo is available through special events or through trading with other players.
Therefore, catching Mewtwo in Pokémon Diamond is not possible without using an AR system to bring in a Pokémon from another game.