>>54170216Why should Celebi be Fairy?
Because it vaguely resembles one?
Celebi is named after celestial being, and the cosmos is strongly associated with Psychic.
Celebi can see into the future with its psychic powers to travel to a peaceful time period. One of Celebi’s Gen 2 moves is Future Sight, and one of its starting moves is Confusion. Celebi is Psychic for a similar reason as Xatunor later on Delphox. They have psychic powers. In fact Celebi is Psychic first and Grass second, showing Game Freak views Celebi as a telekinetic diviner that can also produce and manipulate vegetation.
Kantonian Mr. Mime is only Fairy because it is a trickster. Many Fairy Pokémon like to prank unsuspecting people and Pokémon.