Since every Don is a criminal ["and all the sinners saints...doo-doo," with apologies to Mick Jagger ] I prefer to look at it as:was Michael a successful Don? My answer is no:
Vito understood that he was operating outside the law, and was creating his own law. He would never be accepted in "legitimate" society, but he could achieve the goals he set out for himself: protect his family; achieve stability and a modicum of "rough justice" in his world, among his people; and "refuse to wear the ring through the nose." Michael was not successful. He was an overachiever. He was obsessed with an unrealistic goal: to be considered "legitimate" even though he was the biggest organized criminal in America. His entire life was a chronicle of winning battles and losing wars. He was a failure.