The doctrine of decorum has its modern form in the fallacy of "imitative form." The form of the literary work should somehow imitate the subject-matter. Is this really a fallacy though? In its naive version, maybe. But haven't poets always believed in some version of this idea? "When Ajax strives somes rock's great weight ... " then the verse about Ajax doing this will be, if not imitative, congruent with the idea of slowness and effort. No, you can't write a limerick about the holocaust, I'm sorry. That's an obscenity.