Not to pile on the Calvin and Hobbes comparisons, but I seem to remember Calvin's Dad doing the same thing to get through a loathed reading of Hamster Huey and the Gooey Kablooie.
Lucy's response here demonstrates her developing personality; she is becoming less reluctant to express displeasure.
Another story-reading strip makes a Schroeder & Beethoven joke without actually showing Schroeder. A strip like this doesn't work unless the reader brings into it knowledge from other strips, a type of gag that doesn't work unless the characters have strongly defined personalities. (Or, in Schroeder's case, a strongly-defined personality quirk.)
