B. insulting or scandalous
A ribald is a sort of mocking that is done in a disgraceful manner. Thus, what the narrator is telling the reader is that while, yes, the person being spoken about is funny (as a jester usually is) and can be lyrical by manner of poetry, the topics of which he sings (or writes poetry) about are demeaning because he focuses on sin which is insulting and scandalous.