Anyone not around in the era of '72 may not realize that the recording of MacArthur Park was playing all over the radio incessantly, and was a huge hit for Richard Harris (Dumbledore for some of you, King Arthur for others). Harris had parleyed his success from the Camelot musical film into a mildly successful fame as a tough yet sensitive singer.
The joke punchline in the BC strip was funny at the time because "MacArthur Park is an unusually complex song for pop music. The song was first recorded by Harris on his album A Tramp Shining, in 1968 and was released as a single. It was an unusual single, running for more than seven minutes, with a long, climactic orchestral break."
So anyway, that's the basis of the joke. Well . . . you kinda had to be there.
You can listen to MacArthur Park by clicking here.