A pom-pom alternative to Japan's 25 mm gun

Would Japanese anti-aircraft fire have improved if the navy developed the pom-pom instead of adopting the 25 mm gun?

Probably; the 25 mm was so limited that continued pom-pom development was unlikely to produce a worse result.

Continued pom-pom development is judged unlikely to produce a worse result than the Type 96 25 mm gun. The criticism centers on the 25 mm weapon's short effective range and poor accuracy even at close quarters, leaving aircraft able to deliver their attacks before the gun could do enough damage. Japan nevertheless chose it in the expectation of technical improvement. The counterfactual favors developing the existing pom-pom family, without establishing the performance of a particular replacement design.