Nice to see someone else noticed that too.

In The Godfather Part II, Michael smiles only two times. The first time is when he forced a smile when he and Senator Geary are taking a picture during the party. The second time is when he's at the meeting in Havana. And he may smile a little bit when he meets with Geary during the show.

In Part III, I recall he smiles multiple times as he's a changed man, or so it seemed at least..

But in the entire Trilogy, Michael never really laughs.