Say that I did business with Acme Corporation and my salesperson was John Smith. I want to write a review of my service that specifically says "John Smith was not a good sales person" in my online google review of Acme corporation. Can John sue me for defaming him even though I was reviewing Acme?
No, you didn't make a false statement about him, you expressed a personal opinion which is neither true nor false. Defamation isn't defined in terms of "where you publish the statement", it simply is about publishing a statement, which is damaging, and which is false.