Beetle Bailey: He's been reading that book on positive thinking.

On the one hand, this is the kind of low-key, character-based humor I generally praise. On the other hand, it's not funny.

The problem is that the joke feels incomplete. "He's been reading that book on positive thinking" is the punchline, but it's dangerously low on punch. The obvious, but superior punchline would involve some sort of pratfall in a third panel. It still would have been surprisingly solid character work, and I might have even laughed.