A REAL jolly fellow is Louis Castro as he sparks up the airways every day at mid-morning with his bright and breezy manner. But what a cruel trick was played on him by the elements the other day. Chattering away in his cheerfully bilingual manner, Castro greeted us listeners with: ''You're tuned to FM Select - the yuppiest station in town.'' That's what we think he intended to say. For at our neck of the wood the message crackled over as ''the yukkiest station in town''. Do we blame static interference or Freud?