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

It doesn't. Saying "I am an X because Y" doesn't mean "all X are such because Y".

Even if the character did use the latter formulation, it would be a bit of exaggeration to interpret it as strawmanning. Characters in a joke aren't usually supposed to voice the author's opinions.