Why did Mr. Mime grow an extra digit on his hand after first appearing in the original Pokémon games? Mr. Mime has always been considered one of the strangest entries in the original generation of 150 Pokémon. Mimes are strange enough creatures, but with its clownish hair, dead-behind-the-eyes expression and psychic abilities, Mr. Mime was more of a curiosity in the Pokédex than a creature trainers would actively seek to catch and add to their party. Nevertheless, Mr. Mime's presence in the Pokémon anime and game series allowed for some memorable moments of comedy, and Detective Pikachu's interrogation scene demonstrates this perfectly.
Adding to Mr. Mime's uniqueness and intrigue is the fact he grew a fifth finger shortly after the release of the original Pokémon Red and Blue games. Neither Nintendo nor The Pokémon Company revealed why Mr. Mime's design had been changed, and fans simply assumed that this was one of several slight alterations that were made to several Pokémon as the franchise's graphics became more sophisticated. It has since been confirmed that this is not the case.
As revealed by Jon Sahagian on Twitter, a member of staff at The Pokémon Company confirmed that the real reason a fifth finger was added to Mr. Mime's hands was as a response to complaints from concerned parents. In Japan, the Yakuza gangs have a ritual called Yubitsume, where a member will cut off their pinkie finger and present it to their boss to atone for a misdeed. As such, having four fingers on a hand can be associated with being part of a Japanese organized crime syndicate and, apparently, some parents were worried that Mr. Mime was moonlighting as a hardened gangster when he wasn't busy trying to escape from an invisible box.
Mr. Mime's Pokédex entry in the Gold and Silver generation still has the original 4-fingered hands, but Mrs. Ketchum's Mr. Mime in the Pokémon anime series has the altered extra digit. The change was adopted by the games in time for the Fire Red and Leaf Green releases and is now standard across the franchise.
Certainly, the worries seem a touch excessive. Plenty of humanoid cartoon or video game creations have less fingers than an average human hand but still slip by unnoticed, and Mr. Mime's original design is no different. Perhaps this is simply a sign of how heavily Pokémon impacted the mainstream upon its release, where even the smallest detail was subject to scrutiny from parents deeply worried about how much time their child was spending on their GameBoy. Other strange controversies that emerged around the same period include concerns of Nazi imagery in Koga's gym, the bad influence of the "gambling" mini-games on children and celebrity psychic, Uri Geller, attempting to sue Nintendo, claiming Kadabra was a parody of himself.
The thought of Mr. Mime being a member of the Yakuza is quite Farfetch'd, but it would certainly explain the scene in Detective Pikachu where Mime refuses to talk during Tim and Pikachu's Pokémon interrogation scene.
Source: Jon Sahagian