I think it was his dumbest move, other than invading the USSR when he did. He lost all opportunity to consolidate his gains and the chance to defeat the UK at the same time. If we’d gone after the Japs first, he would have had years to prep for dealing with us. He probably wouldn’t have had to.
Mussolini lost the war for Hitler, he lost valuable time in launching Barbarossa in order to bail out the Italians in the Balkans.