Except Decade didn't actually do this, and you already admitted that Asmodeus would beat him. Zi-O after already getting his ultimate form was still subjected to the narrative powers of Asmodeus. They already encountered each other, and it ended with Zi-O needing help from everybody else. And three people who had the same power as Asmodeus to boot. It took a team effort to destroy him.Except Decade was said to be able to erase a Super Sentai world and Ohma Zi-O scales above a stronger version of him.
This is like saying the Flash could have beaten the Anti-Monitor by himself.
It was not "bad" writing. It was series escalation in order to raise the stakes of the conflict. There was a tangible reason why Asmodeus was this overpowered.