It was a smart move because Barzini and the other 5 families were more powerful than the Corleones and was expected to win. On the other hand, Tessio knew Mike as a weak but pleasant man. Tessio and Clemenza was kept in the dark about what Mike were planning. To be sure neither one expects that he had the nerve to slaughter the heads of the five families.

In the meantime, Tessio was carving a life after the Corleone demise. His main flaw is that Tessio was brown-nosing Barzini in front of Michael plus wanting him to set a meeting with Barzini. Tess didn't realize that Michael saw though this. Therefore, he was left out of the murders and he was killed himself.

You can say that he was not smart because he paid with his life, but those are chances one take