Between this strip and the previous one, we're establishing Charlie Brown's notable lack of skill as a pitcher.
There are a number of stages in establishing a trait for a character:
- First, the trait is exhibited in a joke, but isn't remarked upon. It just happens. The character is being used more as a stand-in for a kid rather than himself.
- Next, the character, after the trait has been exhibited a few times, becomes known for it. This stage comes when other characters begin remarking on the trait as being associated with the character. The remarking shows that Schulz is intent on taking the character in this direction, so he mentions it to it'll stick in the reader's mind.
- Later the character becomes so associated with the trait that Schulz can just use the character as a shorthand for the trait, without even having to explain it in the strip, relying on the reader's past knowledge of the character to supply the punchline. Schroeder is arguably in this state already with his distinctive musical ability, but the other characters are still gelling.
- Finally, the character may become so associated with the trait that the character's presence itself may become a punchline. I don't think we see this for quite a while, though.