I am new to Mochi and seem to have somehow created multiple Flashcard and Notes decks where I have two of each, plus the named deck which I have created. I don't see a way to delete a deck. Surely there is a way to do so. Can someone please help?

You can right-click a deck in the sidebar and choose "Delete Deck" from the context menu, or, while on the deck page click on the ... menu at the top right and choose "Delete Deck" from that menu. This will send the deck to the Trash and from there you can either permanently delete the deck or empty the trash.