I saw that comic yesterday and it actually took me a minute to get it because, although I can (in retrospect) see Ariel as a vegetarian given the Little Mermaid universe, I still have this mental image of mermaids/merpeople appearing as one thing while actually being sharp toothed predators and damn scary. So Disney merpeople I would say eat the first option, and probably the same for other "kid friendly" universes, but for any other version I'd figure omnivores.

(I recognize that it's the communication thing that doesn't translate well to modern humans. We eat other mammals, after all, so fish eating other fish, besides being reflected in nature, shouldn't be such a stretch. But we can't literally talk to our food, and that makes all the difference.)