Yes, it’s a joke. Oral Sadism and the Vegetarian Personality was an article published in the Journal of Polymorphous Perversity, which was sort of like Mad Magazine for mental health professionals. Here’s an excerpt from the article, supposedly a direct (but badly mistranslated) quotation from some German-speaking authority:
The man who kills animals for meat gives the pursued animal a chance to escape. How more and more sadistically cruel is the non-meat eating man. The keen theoretician must himself this question deeply ask—What is the likelihood that the tranquil carrot from its vicious predator successfully outrun can?
The article points out that in paranoid personality disorder, there is, by definition, pervasive and systematic distrust of people, while vegetarian personality disorder involves mistrust specifically related to the content of the dinner plate:
Individual thinks that there are pieces of dead animals on his plate…. In advanced stages of the disorder, individual suspects that there are minuscule animal by-products mixed with his food.