One big misunderstanding about almost everyone in WWII was that everything was well organized top down. Hirohito had only minor involvement in the politics of his country. He only took the throne when he was 15, during/right at the end of a depression and chaotic first democracy in Japan. The previous Emperor Taisho had abdicated due to a mental illness. Hirohito oversaw a time of great political turmoil in Japan. People picture Japanese politics as always being very well organized, but this time was more like the Wiemar Republic in Germany. The infamous Hideki Tojo's Militarist/Facist faction (Toseiha), basically ousted the Imperial (Kodoha) faction that would have been the only one to give the emperor any power. The Kodoha faction basically believed in a Meiji type imperial government, whereas the Toseiha faction pictured it as a Nazi like government. Since the fascists won, there was no chance of Hirohito being involved in politics expect in a figure head type of way. He was a very gentle person, and would go along with most of whatever the most powerful person recommended to him. He was personally not happy about the wars in China. However, he did show public support, but that was basically to the extent of going to a photo-shoot or event in a military uniform. So, it depends on whether you consider making public displays that imply you support the war a war crime. Because he had absolutely no power besides public appearances that could sway people's minds.

Also, Japan knew it usually had very shaky ground in which to invade China/Manchuria ect ect. A lot of what happened to lead up to the war was intentional manipulation by Japan, as Germany did in WWII. What they did was use any excuse to send their military into a foreign area, which was not usually entirely opposed by all the existing political factions of the place they were going to. Then their military would cause provocations and get into skirmishes until Japan declared war and invaded. But all of this was clearly manipulations to find an excuse to invade, and that is what was shown after the war. The military wanted a fight and so did the ruling faction in Japan , and Hirohito did not have anything to do with those decisions and backdoor manipulations.

Also, it is a primarily American thing to blame the Emperor for the war, and that has carried over from WWII. The American people wanted him executed, because they did not understand how little power he had. Also, MacArthur knew that Hirohito was a key to gaining the Japanese people's support in the occupation after Japan's surrender, and he was right. MacArthur became more popular than the Japanese democratic government after the war. Trying Hirohito would have been a big mistake, he had no genocidal intentions nor intentions of leading Japan into a war with anyone, so he was also not a war criminal.

However, remember when you are talking about the intentions of a single historical person, bias and lack of credible sources seeps into what people think. But, no, he had almost no political power except for his family that could venture out and speak for him and his public appearances.