Amagi's loss was a tectonic accident, not a design failure

Was the loss of the battlecruiser Amagi mainly bad luck?

Yes; the Great Kanto earthquake damaged the hull on the stocks, an external geological event that planners could not reasonably design around.

Amagi was lost because an earthquake and associated tectonic disruption wrecked the incomplete hull while it remained on the stocks. No realistic naval design process could eliminate the risk of a major earthquake breaking an unfinished ship in its yard. The loss therefore says little about the battlecruiser's intended performance or the feasibility of conversion into an aircraft carrier. It belongs to industrial contingency rather than a flaw in the battlecruiser concept or conversion plan.