How seriously did Japan rate the American carriers?

How much of a threat did Japan consider the American carriers?

Not much; Japanese planners treated them mainly as defensive assets rather than the leading American naval threat.

The question comes while Enterprise and Lexington are being located around the Pearl Harbor operation. Japanese planners did not regard the American carriers as much of a threat and expected them to serve largely defensive roles. That contemporary assessment helps explain why the carriers' absence did not invalidate the strike in Japanese planning. Their later wartime importance makes the judgment look badly mistaken.