All it really needs for Mussolini to be seen as a lot more as an evil villain is for public perception (in Europe & North America) to care more about lot of people dying in continents other than Europe. For a start it would be nice to see the movie Lion of the Desert to be a lot more financially successful than OTL, to spark some interest in the actions of interwar period Italy.
Even then, Mussolini didn't kill nearly as many people as Hitler did, and what he did is very easy to wrap up under the crime of colonialism, where such mass murder was frequent. Although I could see exaggerations and undue attention paid to Italian crimes by other European nations and the US for the purpose of minimising their own crimes committed in Africa and Asia during colonialism, which would obviously draw a lot of attention to what Mussolini did in Africa. "No, the British Empire ruled beyond benevolently in Africa, unlike those awful Italians!"
As for Hitler to been seen as a bumbling fool, just have the POD where France & Uk declare war on his arse as soon as he put troops in the demilitarized zone. Either he backs down (and future internet historians will call him a bitch), or Germany get's pounded so hard the resulting little known documentary made decades later ends up on pornhub as a lark. Either way his cred takes a major hit.
How many people outside of Germany would even know who Hitler was if he was defeated at that point? Maybe Czechoslovakia decides to go down fighting in 1938 when the world ignores their plight, and Hitler's army is thrown through a meat grinder with tons of casualties before Czechoslovakia finally surrenders. Pop culture will tend to ignore the significant advantages 1938 Czechoslovakia had (starting with their fortifications) and focus on how a Great Power like Germany got their teeth kicked in repeatedly by a much smaller country (much like the Winter War). In 1939, Hitler goes to war with Austria over Anschluss, and gets his teeth stomped in by a Franco-British intervention which is over by 1941. Austria holds out for several months and scores several notable victories over invading German forces. So basically, Hitler is that one German guy with the funny mustache who talked a big game and came up with grandiose plans for a new empire, but his army could barely even get past weak opponents like Czechoslovakia and Austria.
The problem is making Mussolini a lot more evil than he was and fixing the deficiencies of his armed forces to allow the world to suffer more from it. Maybe he still conquers Ethiopia, and his regime is unmatched in its brutality due to repeated resistance from the Ethiopians and well-publicised attacks on Italian settlers there. Maybe make Mussolini more racist against Africans and bring back slavery in all but name--make it as bad as the second coming of Leopold's Congo, something that makes every British and French African colony look like paradise. Finally, Mussolini goes for conquests in Europe, and ends up conquering Greece and Yugoslavia in quick campaigns that go off without a hitch, in a way that makes the Greeks and Yugoslavs look utterly incompetent. The Regia Marina does far better than OTL. The Italian Army then fights in Africa, invading Egypt and Tunisia while attacking France in the south. In Asia, they commit support to the Japanese, and eagerly declare war on the US when Japan and the US fight. Overextended on every front by late 1942, the loss of their Libyan oil leads to their gradual collapse in 1943, although Italians are still fighting tenaciously to defend their homeland. Victor Emanuel is deposed in late 1943 to prevent Mussolini's arrest and removal, and he is accidently killed during the coup. By 1944, Allied forces have surrounded the last few holdout Italians (all fascist partisans) in the mountains of Central Italy, where Mussolini shoots himself in a bunker.
Mussolini's reputation and legacy is one of great evil, an example of a man who drenched the world in blood to build himself and his country a totalitarian empire in some insane scheme. Like the Nazis, his military is overexaggerated in their abilities, and people regularly imagine Italian forces marching through Paris or Jerusalem.