The decline in Walmart's sales is a direct result of an improving economy. The increase in Walmart sales of the past five years was driven by a sagging economy forcing people to shop for lower priced items than they would have otherwise purchased. As the economy has improved shoppers have gone back the higher cost items.
There is no reason for them to bring back "happy faces." Walmart is a multibillion dollar corporation and while they may suffer a bit recently, there is no reason for concern. They will continue to rake in insane loads of money. Just like everything else though, when the economy is down, so are they.
Much of the decline that has plagued WalMart's business can more likely than not be attributed to online shopping. People are able to shop a virtually unlimited marketplace, compare prices, and read reviews all without leaving their home. The issues that are facing WalMart are likely being faced by virtually every other big-box store, and bringing back the happy faces will not result in increased revenue.
I don't think the disappearance of the happy faces have anything to do with their declining sales. I think they are having a hard time competing with Amazon and other Internet vendors. They will have to re-evaluate their business model if they want to stay relevant in today's economic world.