Mueller shouldn't be making any comments regarding impeachment, he's not a member of the House (nor can he act in a capacity as a private citizen), he's an acting law enforcement official. The House is the sole arbiter of what rises to treason, bribery and other high crimes and misdemeanors in terms of impeachment.
The Democrats are already trying to quash talk of impeachment because it'd go nowhere, if Mueller came out and said there was no evidence of Trump doing anything worth charging, they would shut that shit down even harder except for the fringest of fringe members.