Was Nagumo too cautious to command Kido Butai?

Was Nagumo naturally unaggressive because he was not a carrier specialist?

No. His career as a highly aggressive destroyer officer argues strongly against that characterization.

Nagumo's destroyer background weighs against describing him as naturally timid. Destroyer officers were selected for the willingness to bring small ships close enough to deliver torpedo attacks. He had risen to senior rank in that demanding specialty and is judged here as one of Japan's more aggressive destroyer commanders. His later carrier decisions therefore need to be assessed against their operational circumstances rather than explained simply as a lack of aggression.