I think he was giving Kay a tour de force on the family in that scene. Since he didn't consider himself part of the family ("That's my family, Kay, it's not me"), I thought he was speaking as something of a disinterested outsider, so I don't read disapproval into "not a Sicilian."

On the other hand, I read into it that Michael had already spotted Tom's weakness and summarized it brilliantly: "He's a good lawyer--not a Sicilian." A good lawyer could grease the wheels for the family's "legitimization" in Nevada. A Siclian would have smelled a rat re. Carlo and Barzini.