Yamamoto Isoruku was an energetic, determined and charismatic leader. However, the problem with calling him a genius is that his judgement, particularly of American attitudes, tended to be very poor.
The Pearl Harbor attack was certainly Yamamoto's idea and he possibly started planning even before the British attack on Taranto. Yamamoto realised that the IJN could carry out such an attack "successfully" despite the doubts of other IJN officers (and also the general American belief, except for Adm. Richardson, that it was not logistically possible).
There are serious criticisms of the military planning of the Pearl Harbor attack made for example by Alan D. Zimm in "Attack on Pearl Harbor: Strategy, Combat, Myths, Deceptions", which points out that there was little attempt to protect the bombers either from anti-aircraft fire or possible fighter attack and that there was little overall plan of target selection.
However, the worse problem with the Pearl Harbor plan was that it made it almost impossible for Japan to make a subsequent compromise peace with America. Yamamoto was warned about that by Onishi Takijiro in August 1941, who argued "...we should avoid anything like the Hawaiian operation that would put America's back up too badly" ("The Reluctant Admiral: Yamamoto and the Imperial Navy" by Agawa Hiroyuki, page 229) but ignored Onishi. Interestingly, Yamamoto actually planned the attack with an eye on American opinion according to Minoru Genda, who is quoted as saying "Yamamoto wanted to target only battleships. He appreciated the importance of destroying aircraft carriers but believed that the psychological effect on the American people of destroying all the American battleships would be greater than that of the destruction of carriers." (I found this in " Winning" the Pacific War: The Masterful Strategy of Commander Minoru Genda by Angelo N. Caravaggio
https://usnwc2.usnwc.edu/getattachm...the-Pacific-War--The-Masterful-Strategy-.aspx where it is referenced to Minoru Genda, interview by Gordon Prange, 9 March 1948, interview no. Minoru 2A, transcript, Prange Papers).
The other obvious point about the Pearl Harbor attack was that Japan could not try to attack only the British and Dutch possessions. I am slightly hesitant to discuss this because I have never convinced myself that I understand who wanted to attack the Philippines in 1941. However, Yamamoto clearly saw no advantage in trying to avoid American territory. There is a brief mention of a war game on 26th-28th November 1940 presided over by Yamamoto with "leading members of the Naval General Staff" in "From Mahan to Pearl Harbor" by Sadao Asada (page 279). The conclusion was that an attack on the Dutch East Indies would involve war with America. I am not sure how that conclusion was reached but I suspect that Rear-Adm. Maeda Minoru was involved. Marder's "Old Friends, New Enemies" mentions an interview with Maeda from 1962 in which he claimed responsability and that he had used the argument that the Dutch and the Americans were the same race! Yamamoto was probably correct about America entering any war by late 1941 but I am not so sure about 1940 or that he had the right reasons or that he was thinking beyond the start of the war towards possible negotiations.
As well as posible political errors, Yamamoto has also been criticized for his planning as Combined Fleet Commander. I have not read Mark Stille's short biography "Yamamoto Isoroku" (except the extract at
Yamamoto Isoroku) but I suspect the comment "As with any famous military commander, there is always reality and myth. For Yamamoto these stand in stark contrast." is inspired by doubts about his military rather than political judgement. Certainly, Yamamoto did not feel bound to keep the six carriers of the First Air Fleet (Kido Butai) together, which almost everyone with the benefit of hindsight supports. Interestingly, according to Mark Peattie's "Sunburst" it was not Yamamoto but Ozawa who had been the early advocate of concentrating Japan's carrier force (
Sunburst).
However, whilst it is obvious that the plans which divided the carriers for Coral Sea and Midway (and earlier without a carrier battle around Wake Island) were very bad, I can see a crazy logic for the Midway plan at least. I believe that Yamamoto assumed that the Americans would not fight unless they were offered a very tempting target. Thus, he was happy to send two carriers to the Aleutians and spread his battleships across the Pacific in order to offer such a target. As Nimitz had sent two carriers to fight against up to five Japanese carriers at Coral Sea (Kaga was believed to be attached to the 4th Fleet on the basis of early decrypted signals), Yamamoto again failed to understand his enemies or learn from recent history.
An earlier example may show the same failure to understand America, although I am not at all sure if Yamamoto was deeply involved. However, Yamamoto was certainly keenly interested in the development of long range bombers, especially the G3M. Indeed, that is the only major technical innovation in which he played a major role. What is less clear is whether he, at the time Vice-Minister of the Navy, supported the IJN's use of the G3M from the start of the 1937 conflict in attacks on Chinese cities. Photographs of those attacks, which naturally were chosen to show civilian casualties, did little to improve Japan's image, especially America with its missionary linked China lobby.