No, Japan was not a dictatorship. It was not significantly better though, and is best considered a khakistocracy if anything. The main point of contention is that Japan did not have any single political leader with absolute authority, rather the entire military assumed the role of authoritarian government, together with non-elected civilian policymakers. In case this sounds like a monolithic entity, it was absolutely not.

By 1937, Imperial Japan was essentially led by the Liaison Conference. This was a tripartite assembly of the civilian government (Prime Minister and Foreign Minister), the Army (Minister of Army, and Chief of Army General Staff), and the Navy (Minister of Navy, and Chief of Navy General Staff), presided over by the Emperor. This style of government was notoriously volatile and certainly not dominated by any single person, least of all the Prime Minister.

As a case study, we can consider PM Fumimaro Konoe, who tried very hard to achieve peace with the USA. In April 1941, Konoe presented to the Conference a proposal by Roosevelt which would essentially recognise Japan's claim to Manchuria, and its status as an equal (including immigration rights). He believed this could be the starting point for fruitful negotiations that could terminate the war on very favourable terms, and prevent further risky bloodshed. The Navy was originally supportive, while the Army wanted to pursue further gains in China. With the balance tilted, Konoe was able to have Roosevelt agree to two peace summits in Hawaii and Alaska which could have ended the war.

In July 1941, the US, Netherlands, and UK imposed a blanket oil embargo on Japan. This meant that the Navy had only 22 months of fuel remaining, a stressful situation which meant the Navy suddenly became very open to invading Southeast Asia to secure it's oil supply rather than sue for peace. Konoe was now outnumbered in the September 1941 Conference, during which Army representative Hideki Tojo argued that if Konoe could not secure a peace deal in 20 days, then peace was to be abandoned. Konoe immediately tried to convince Roosevelt to submit a better leave proposal, but Roosevelt believed that Konoe was bluffing. On 14 October 1941, the Tojo proposed escalating the war in China, and this time the Navy also agreed to coordinate activities in the Pacific. Now 2-against-1, Konoe resigned 2 days later knowing that he had failed to achieve peace.

These 3 factions were essentially locked in a power battle with each other, just as much as they were at war with the rest of the world. Even after Tojo became the PM, in Sept 1942, the entire Foreign Ministry threatened to resign in protest against the establishment of a Greater East Asia Ministry. Between the Navy and the Army also sat a deep-seated rivalry, which meant that apart from the Imperial General Headquarters, which met twice a week to discuss proposals, there was almost no communication between the two services. Finally, we have the Emperor, who was not a figurehead and had genuine political sway, but was aware that his role was mostly to preside over and guide the decisions of his Cabinet. Consequently, Hirohito was (at least initially) a well-known pacifist, who Konoe believed was only very gradually brainwashed by Army/Navy representatives, and otherwise single-mindedly desired peace.

Bix (2010) - "Hirohito and the making of Japan" Jansen (2000) - "The making of modern Japan" Oka (1983) - "Fumimaro Konoe" Falk (1961) - "Organisation and Military Power"