Getting back to the original topic, the writers put some nonsensical words in Michael's mouth at the end of GF, and these are among them.

Describing Tessio as smart for signing his own death warrant by falling into Michael's trap is like calling Vito smart for sending Luca to the Tattaglias.

A smarter person would have seen Michael's talent and intelligence after he figured out the right way to kill Sollozzo. A smarter person would never have believed that Michael would really make Carlo his right-hand man. A smarter person might have listened to Michael when he told the capos that negotiations were under way to solve all their problems and answer all their questions. A smarter person would have thought twice about approaching Michael with a fake meeting shortly after expressing doubts about the future of the Corleone Family - in front of Michael, no less.

Maybe Clemenza was dumber, but Tessio was hardly smart.