The point is that we don't have to agree what good is to agree that things are bad.
And its relevance is that I don't think that anyone is trying to claim that the Kingpriest was other than bad. The two camps appear to be:
- The Kingpriest was evil and corrupt and, given that good and evil are metaphysical forces in older D&D clearly was Evil.
- The head god of Good says that the Kingpriest was good. The statblock says the Kingpriest was Lawful Good. By Dragonlance standards the Kingpriest was, therefore Good. Which means that there is something deeply wrong with Good on Krynn.
The second option is the one I find to be (a) correct and (b) far more interesting for worldbuilding.