"Funny" is subjective, so pointless to argue about that. Of course it's oversimplified, it's a web comic!

Agreed.

those who do it for ethical reasons certainly consider sentience an important factor.

No, some of those who do it for ethical reasons consider sentience an important factor. The comic both strawmans all vegetarians as ones who do it for ethical reasons, and assumes that those ethical reasons are because they don't like killing "conscious" animals (whatever that is defined as). As a vegetarian (which I define as anyone whose utility function contains a negative term for eating meat), neither of those are true for me.

The author suggest one simple metric: minimize sentience/calorie, and makes a pretty convincing argument, as webcomics go. You know, an actual calculation, not just emotions. Seems like a much more rational approach than demonstrating against killing baby seals because they are aww so cute.

An actual calculation is probably a lot worse than your intuitions, when you oversimplify the neurobiology as much as the author does. If ey had actually gone into a discussion of glial cells versus neurons, labelled line vs across fiber organizations, etc. then ey might have a valid argument. But the author commits illusion of transparency when ey assumes that animals that move slowly and are slothful are the "stupidest".