Did treaty limits force poor heavy-cruiser designs?

Can the 10,000-ton treaty limit be blamed for the problems of interwar heavy cruisers?

Only partly. If a navy was already exceeding the limit, the treaty did not require it to preserve an unbalanced design.

The treaty ceiling made heavy-cruiser design difficult, but the Mogami class was already understating its displacement. Treaty compliance therefore cannot explain every missing capability. Once a navy was exceeding the limit, additional weight could have produced a more balanced and protected ship. The external constraint was distinct from the priorities designers chose within and beyond it.