That's not common at all, stop making shit up. Most people, vegetarian or not, are not as fucking incompetent or lazy as the Onions, at least not in adulthood.
No doubt their shitty diet causes a lot of their physical and even mental problems, but it's not a common thing. It's because Gurg and Lame are abnormally retarded.
Those poor kids though. Kind of wish Billie did let Gurg run off so Lamey would move back with her parents who might make them some decent meals.(necromancy)