A possible meaning of the metaphor would be very disappointed because "dirt disappointed" is not meant in a positive sort of way. If you're dirt disappointed it most likely means you are very disappointed, possibly aggravated. Hope this helps
Poe dresses Fortunato as a fool because he followed Montressor into the catacombs in hopes of sampling a rare bottle of wine which are the actions of a fool. In the end he allows Montressor to manipulate him and kill him without realizing the real intentions of his "friend" until it is done
