Clicking a link can't infect your computer with a virus anymore. As long as any vulnerabilities in QR scanner apps are reliably patched, it should be just as safe as clicking a link. There are zero-days, but that risk exists any time you're connected to the internet anyway.

The cost and risk of putting a sticker on a wall is much greater that that of sending a spam email. Legitimate advertisers already hire people for >$0 to do that. Illegitimate ones risk personal criminal prosecution because they have to be physically present.