Jack Applin sends:
He asks, “Is the carrot implying that he eats only carrots, and that the donut eats only donuts? So Billy is a cannibal? Or, does Billy only eat other Billys? Why would either of those possibilities bother a talking carrot who is a cannibal himself?”
Comics I Don't Understand 
Maybe the carrot and the doughnut used to be humans, then transformed according to the YAWYE rule. Billy, however, by remaining (or re-becoming) a human, reveals his past cannibalism.
Jack got the “joke”, it simply does not land.
I was assuming Billy was breaking the 4th wall and wanting to eat readers. I like Mitch’s explanation.