From Muppet Wiki

Masterpiece Theater is a long-running PBS anthology series, originally hosted by Alistair Cooke. The series was regularly spoofed on Sesame Street as "Monsterpiece Theater". The latter borrowed the original's theme music, and even changed its opening titles to reflect changes to the original Masterpiece.

Though most of the "Monsterpiece" sketches ranged from classic literature to popular films and plays or even television shows, some of the earliest sketches spoofed actual Masterpiece Theater series, such as Upstairs Downstairs and I, Claudius.

Masterpiece Theater was among the PBS shows mentioend by Kermit the Frog in Sesame Street, Special when trying to find reasons for Oscar the Grouch to like public television.