Astute question, Lana.

I can't prove it--I'm
inferring from the number of times we see Michael being abrupt and harsh with Tom, and his statement that "You're not a wartime consigliere," that he doubted Tom's competence. So, I believe it's
possible that he might have been inclined to blame Sonny's death on Tom because Tom didn't see Carlo's treachery coming. It's also
possible that Michael
may have resented that he had to take the reins of the family following Sonny's death, and needed Tom as a scapegoat. Likely? No. Possible? Yes.
Nothing changes the fact that Sonny dug his own grave through his uncontrollable temper.
Thanks Turnbull
Tom was a good lawyer but not in the
consiglieri league even in 'peace' time but there were no other contenders!
Tom's "competence" was poor indeed and though Michael's "You're not a wartime consigliere" was in conjunction with Vito, the way it was delivered by Michael was in stark contrast to Vito's “I never thought you were a bad consiglieri....
Then Michael's curt dismissive “You're out, Tom†to Tom's “Maybe I could helpâ€
“the number of times we see Michael being abrupt and harsh with Tom†poor Tom even humiliating Tom in front of Ola, Neri and Rocco - using and abusing Tom the “only one Michael could completely trust†Go figure!