MTV Networks announced Thursday that its chairman and chief executive, Judy McGrath, was resigning. The surprising change at the top comes at a time when MTV is riding a surge in both ratings for its programs and profits for its parent company, Viacom. Viacom said Ms. McGrath had decided it was a perfect time to leave to pursue other ventures, although those ventures were not named. She leaves at time when the company has ascended to a new peak of success, with programs like “Jersey Shore” and “Teen Mom” on MTV, “SpongeBob SquarePants” on Nickelodeon, and Jon Stewart and Stephen...