"Do you have to have a low IQ to smoke cannabis in the first place?"

No! You just have to be a conformist and bow to peer group pressure.

You assume a lot. I did smoke pot but not due in any way to peer pressure. In fact I had to start hanging out with a different group of friends when smoking became my preference because the folks I had been (mostly) hanging with did not smoke. Many in the new group were already my friends but they were not the folks I (mostly) hung out with until I started smoking.
Peer pressure had zero to do with it.