This was a term coined during what some consider to be the best vintage VGA episode, Let's Play Ballerina. While there is no widely accepted consensus of what the term actually means, it's assumed that anyone, or anything can be So Ballet if he/she/it possesses a "valley girl" accent and is very sassy. For example, Zelda (No, not THAT Zelda!) was considered to be "so ballet" during the Skyward Sword playthrough.
So Ballet is just one of the many voices used on the show. For more, see Voice Acting.