The last two verses of the St. Anthony poem reflect that the fish react the way people would have reacted had they been willing to listen to the sermon. Is it cynical? Absolutely!
The pikes remain thieves,
The eels great amorists;
The crabs go backwards,
The codfish stay fat,
The carp gorge a lot,
The sermon's forgotten.
The sermon was a success,
They all remain the same.
Fatalism was understandable to those who were at Ground Zero for the religious wars that followed Luther's Reformation for over a century. Not until the Treaty of Westphalia (1648) did the German-speaking world know peace and enough tolerance that Catholic and Protestant could live in each other's presence without a desire to murder the other.
The poem remains a work of its particular time.
The Germans just have a general tendency to morbid thought. If you get hold of the complete Grimm's Children and Household Tales, the number of creepy little stories about bad children and their awful fates is surprising. Sort of like Cautionary Tales, only they were serious.
The one my kids called for was the one about the little child who was bad and hit his mother - God was displeased with him and allowed him to fall sick. After he died, the arm with which he struck his mother would not stay in the grave, but kept sticking out of the ground. The mother had to go to the grave and whip the arm with a switch, then at last the child had rest under the earth.
For some reason the kids LOVED that story. That and the one about the little white snake who came out and drank the little girl's milk.