I'd wonder about causation. Does a vegetarian diet drive you crazy - or are crazy people more likely to eat vegetarian? Ritual restriction plays a big part in lots of mental illness so it's a tough one to separate. The study itself draws some pretty pure points - of the people they surveyed, the vegetarians were less healthy. Extending that out into some bombastic Daily Mail shitshow is inappropriate but drearily predictable. More often than not, you can improve your health by eating less meat. Improving your health by eating no meat is trickier. I generally eat beef only once or twice a week and try to go for a meat-free day once a week or so. I'd eat a lot more fish if I didn't live in Los Angeles, which has exactly zero understanding of fish. As in most things, pragmatism is your best approach, I think.I did note that the study was of Austrians - although among the study's sources is a study of Australian women that also finds that vegetarians tend to have higher rates of mental issues.
So, not the Australian study, but there was a study of German participants, and they found actually that participants were likely to have mental instability before adopting a vegetarian diet. I like the Michael Pollan approach. Meat as a side dish, not a main; less meat, but not no meat.