Okay. So Sliph Co. had some people in it who were secretly trying to develop genetically enhanced pokemon for Team Rocket (under threats from the evil team), and one of the pokemon was this Mime Jr. It had been hatched with an unusual strain of pokerus, and they wanted to harvest it somehow. One of the scientist, a female, felt sorry for the poor thing. So she let it out of it's cage to feed it, but it used it's psychic powers to turn the woman into a clown! All her stress left her, and she set the Mime Jr. free.
It came to Sabrina, who (after Ash changed her life for the better) was trying to think up a good comedy routine for tonight - ever since her change-of-heart, she's been trying to purge away her evil reputation by making others laugh. So far, the psychic gym leader had success, but she was drawing a blank tonight. Before, she had had visions of a visitor who would give her the comedy edge she needed, but she did not know who she should wait for.
Suddenly, she heard a voice in her head: "I can help you, Sabrina." it said "I can give you what you need." Using her telepathy, she traced the source, and found the Mime Jr. She picked it up, they touched foreheads, and exchanged memories (a pokemon version of a Vulcan mind-meld ), and once Sabrina knew what it had in mind, she willingly accepted it. She was a big hit that night - everyone loved the new clown gym leader. She had the gym redecorated to look like a circus tent. As challengers came, they battled for badges and entertainment, and if she could sense they wanted to be clowns when she read their minds, she granted their wish, and even if they lost, they left happier.
One challenger was Lyra - she'd already beaten the Johto gym leaders, and now she was going through the Kanto region to collect gym badges here too! By the time Lyra won, she realized she'd been turned into a silly clown! Sabrina told her that Mime Jr. had done all it could here, and wanted to travel to other places and make others happy. So with her new friend, Lyra said her good-byes to Sabrina, and peddled away on the unicycle that used to be her bike, and set off to be the funniest pokemon mistress (female pokemon master) in the world!
Sorry that was so long - I have too much imagination for my own good!