Is there a difference in quality between a hardcover book offered at WalMart (I have heard these described as "mass" hardcovers) and the same book sold through a bookstore?
There are numerous types of bindings and methods used to produce a hardback book.
Discount hardbound are normally bound using the "Cardboard Article" method. In essence, this is nothing more than a paperback with hard covers. The spine is glued as on a paperback.
One step up from this has the spine covered but not the hardcover spine area is part of the covers and not glued to the spine.
Whether paperback or hardbound, most books today use "Perfect Binding." This means the pages are cut and glued, rather than stitched.
As to whether or not the books sold in Walmart have a different binding as the same book when purchased from a bookstore. It is highly unlikely a publisher would risk an extra run or set-up for a different binding.
That being said, when one considers the number of Walmart stores, they could easily be buying a complete production run of a novel, bound exclusively for Walmart. However, most of the books I've seen in Walmart have the exact same binding as purchased from a bookstore or book retailer.
The real-world answer is "it depends".
There are too many variations on book imprints to answer your question correctly. It depends on the book, the size, the publisher, the production run, the market, the outlet, the edition, and a half-dozen other less-important things.
But, in general, "mass market" refers to paperbacks, not hardcovers. The only difference in a mass market book, quality-wise, is paper quality. Mass market books often (but not always) use a slightly lower quality paper.