Are you talking about the report that Trump ordered McGahn to publicly say Trump did not obstruct justice, to which McGahn is said to have refused?
Mueller, a lifelong Republican, specifically states that if he thought President Trump did not obstruct justice he would say so. He said that he can not exonerate the President of having committed a crime and since he can not indict a sitting President he would only lay out the facts. Which he did. McGahns opinion is not a fact and is as relevant to the report as Kermit the frogs opinion.