Why mail-plane catapults were the wrong operational model

Did Germany's mail-plane catapult experience validate Graf Zeppelin's launch system?

No. It proved single-aircraft launching, not the speed, volume, turnaround, and resilience required for carrier operations.

Mail ships and cruiser seaplanes proved that pneumatic launching could work, but they operated individual aircraft with ample maintenance between uses. Success meant delivering one aircraft and its mail. A carrier had to launch a combat formation quickly, repeat the cycle, and keep working through bad weather or damage. The mail service was technically related and operationally misleading as a precedent for Graf Zeppelin.