The plural of “memento mori” is “memento mori.”
Originally I thought the title was trying to be clever (expected something about breath mints, perhaps).
Nice to see The Baffler still kicking around, though!
Edit: the more I think of it, the more I think it's okay -- it is just a play on the phrase and our borrowed noun "momento". Objects that have died in relevance in some way, rather than things that remind us of our mortality directly. Surely the use of Latin in English is not too far from once ubiquitous and now largely forgotten ashtrays.
If you're writing at a desk, look about you -- what do you see? How much of that would be recognizable in 1, 2, 5 generations (about a century) hence? How much would be intelligible, but quaint (or benighted)? Ultimately the little essay is about ourselves, our own mortality: it is about us as ephemera.