It seems to me that Japan and Germany have went out of their way to educate their children around the actions of their forefathers in WWII, where Italy seems to have come out of it fairly unscathed in the face of modern popular opinion.
As far as I remember, Mussolini's reasoning was a direct attack on the democracy and freedom of western nations. Is there a reason for that people seem to have forgotten Italy's role in WWII?
Compared to their allies they didn't have a large effect on the war, their army had few victories, they surrendered fast, and they were not as vicious against the Jewish and foreign populations. They didn't actually declare war until June 1940, the same month that Germany signed an armistice with France and by which point Japan had control of large swaths of China. Mussolini didn't even want to be a part of it, he stated at the time: 'I only need a few thousand dead so that I can sit at the peace conference as a man who has fought'. They also ended the war early in September 1943 with a relatively unscathed country compared to Germany which was decimated and Japan which got to experience the atom bomb. Concentration camps for Jewish populations were only brought into existence after Italy had joined the war and only for Jewish refugees, in 1943 it was still possible for an 'Italian citizen of the Jewish race' to live in Italy^1. So given this, a combination of not having a great effect on the war, not being wholly invested in the war, not taking part in the worst atrocities of the war, and not being affected by the war as much as her allies is why Italy does not share the stigma of Japan and Germany.
Source(s): Levi, Primo. (1979). Introduction. In: If this is a Man. Great Britain: Abacus . 7.
Although I agree with the other commenter that Italy doesn't have as large of stigma because they "didn't have a large effect on the war", I believe that the answer is a bit more complex than the timeline of events. As I see it, there is a historical context answer to this question and academic/historic one.
From an academic/historical perspective, Italy doesn't have as much of stigma because of the relative lack of atrocities when compared to their axis cohorts. Whereas the Japanese had comfort women and the Rape of Nanking and the Nazis had the Holocaust etc., the Italians didn't have a concentrated, bureaucratic effort at killing. Thus, whenever anyone learns about World War II, they learn about the atrocities and invasions, of which Germany and Japan accounted for most.
The other answer, the historical context answer, from at least the American public perspective, is very interesting and far-less known. Throughout the anti-Italian propaganda was relatively low compared to anti-German and anti-Japanese. The reasoning behind this was not simply because "Italy didn't play a large role in the war". We must remember that British-American ground efforts were largely against Italian troops or in Italy herself. Rather, there was a concern in the American government about the effects of anti-Italian propaganda in the United States. There was some concern in World War I about German immigrants, there was much less during World War II. German-Americans were largely integrated while the Japanese-American population wasn't, they were nominal in number and in power to the point that the US government was able to put them in internment camps without much public outcry. However, this wasn't the case for Italian-Americans. Large number of relatively recent Italian immigrants lived throughout the United States and the political repercussions of demonizing these people wasn't lost on the US leadership. Therefore, the narrative of the war and the way it was shaped in the average American's mind was much different. It was a concerted effort and I think it affects our contemporary attitude towards Italy's involvement in the war.