I like it whenever Schulz draws that bust of Beethoven, for how it breaks the art style, but more interestingly than that....
Take a look at that smile the statuette is sporting in its panel. Rather mysterious!
The bust of Beethoven works great as a punchline because it only has to be drawn once. For someone on a comic strip's deadline, reproducing it in detail, and consistently, would be difficult across a whole strip.
That doesn't mean he doesn't do it later on though, and in a Sunday at that....