On an ethical level, it would depend on your own position on responsibility I guess, so that isn't something I feel one can give an objectively true answer for.

Complicity on a legal level, I'd say probably not. Obviously given his death in 1943, he was not able to be tried but I can take a stab at a counterfactual of Yamamoto being tried by the IMTFE.

Given the vital role he took in planning the Pearl Harbor operation (among others), if he was alive to be tried and the IPS decided to indict him, he could likely be found guilty of crimes against peace. As it happened, Admiral Nagano was indicted for his role in planning Pearl Harbor (and likely as a substitute for Yamamoto), although he died during the trial.

As for crimes against humanity/war crimes as far as I understand, Yamamoto had no direct relation to any particular occurrences of these. His role was very much a military command role, and barring evidence that he knew or should have known of particular atrocities conducted by forces under his command, it's difficult to see him being found guilty. There is a potential argument on the use of 'hell ships' but that would require evidence that he was complicit in that policy or had knowledge and control over that area.