I think it depends on the Wal Mart. I bought my 3 Pictus Catfish at Wal Mart last weekend. All their tanks were clean. I can remember seeing no dead fish in any tank. And the Bettas water was clean. The lady netting the fish didn't seem all that knowledgeable about what she was doing, chased them around the tank like crazy, but whoever is in charge of the fish area, at least at this store, is doing a pretty good job it seems.
edited to add -- I must say though that this has not always been the case, even at this store. So I think it just depends on whoever is managing the fish area