Clefairy was originally supposed to be the mascot, but, at the last minute, they changed it to a Pikachu because Clefairy was too shy and Pikachu added more conflict in how they got along and eventually grew on each other. I'd say it was a good move. Pikachus seem very well-rounded in terms of the environment they can tolerate and, yeah, I agree with Aquadon, it can be cute, but tough. Though I'm sure many pokemon can fill that gap if possible, Pikachu was just simple enough, yet not a super common pokemon such as a rattata or a pidgey, to work for what the show needed.