While the U.S. economy expanded a hefty 4.6 percent in the second quarter, growth has averaged only 2.2 percent since the recession ended in June 2009, and that's none too impressive, says Nobel laureate economist Joseph Stiglitz of Columbia University.

"The U.S. has been moving along in this very mediocre way," he told CNBC. "What's remarkable is how low the growth is in spite of the fact that . . . we have some very strong positives," such as the energy and technology sectors.

"The real issue is that, as in Europe, monetary policy is not the key driver" of the economy, he explained.

"What we really need, both in the United States and Europe, is fiscal stimulus."

Stiglitz shares the concern of some Federal Reserve policymakers that the dollar's strength — it hit a six-year high against the yen and a two-year peak against the euro last week — will weigh down the economy by stifling exports.

So will the economy be strong enough to warrant a Fed interest rate increase by mid-2015, as the consensus predicts?

"Almost surely no. Labor force participation is very low, [and] large numbers of Americans would like a full-time job and can't get one," Stiglitz said.

He is concerned about growth elsewhere in the world too — a worry that is shared by many others, including Christine Lagarde, managing director of the International Monetary Fund.

"In the face of what we have called the risk of a new mediocre, where growth is low, and uneven, we certainly believe that there has to be a new momentum," she said at a news conference Thursday.

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