Are crimes caused by Round-Up? I don't know, but I suspect it's a pretty minor force. I would worry more about the lead we pumped into the environment that has "dummied down" the last couple of generations. Not to mention other assorted contaminants that are known to impact brain development. I am SURE I used to be smarter.
But, I am not convinced stupid people commit more crimes. In their own way, there are some pretty smart criminals out there. I would put more blame on people having fewer opportunities to succeed in a more socially acceptable manner. How about we try paying a higher minimum wage? Or providing basic necessities at a cost everyone can afford? Provide free education for better jobs and automate the rest. You know, a Star Trek kind of world.
We would still have some crime, but maybe a lot less.