In "The Cask of Amontillado," are the catacombs a symbol of Montresor's state of mind?

You can certainly make the argument that the catacombs are symbolic of Montresor's state of mind. I like the comparison, and I think it works on a physiological level too. Catacombs are generally made up of a web of interconnected passageways that then have little storage "boxes" along the way or in special rooms. The human brain is similar in construction with all of the neurons, axons, dendrites, and sections of the brain itself. Impulses travel along the physical nerve cells and connections, and information is stored in various parts of the brain. Often, catacombs can be dark and twisted locations, and it's not unheard of for people to get lost in them. I would definitely argue that Montresor's mind is equally "twisted" and lost. He buries a friend (former friend) alive because Fortunato insulted him. Montresor doesn't feel any guilt over what he did either. It's like he's capable of locking down that particular emotion, and that is comparable to how he locked down Fortunato in the catacombs. If Montresor wants to, he can go down and visit the physical location where Fortunato is being kept. That again is similar to how Monstresor's brain works. He is capable of visiting the stored memory of the event any time he wants to, and that is the wonderful story that we get to read.

I think that is possible, yes. Ultimately, Montresor walls Fortunato into a hidden recess at the far back of his family's catacombs, and it seems, some fifty years later, that he has been unable to forget the man whose life he took. In this way, then, just as Fortunato has been imprisoned within the walls of Montresor's catacombs, Montresor has been unable to forget about what he did to Fortunato all those decades ago. Fortunato, in a way, remains trapped in both: the catacombs and Montresor's mind.
Also, in the beginning of the story, Montresor describes the vaults as "insufferably damp" and "encrusted with nitre."  As a result, they will be especially dangerous to Fortunato's health since he is suffering from a severe cold. Montresor's mind—with its murderous inclination—is also certainly dangerous to Fortunato's health. In these two ways, then, the Montresor family catacombs could be seen as symbolic of Montresor's state of mind, both during the events of the story and long after they took place.