The unemployment rate just dipped to a 50-year low, but Nobel laureate Robert Shiller says the number may not be the best indicator of the strength of the job market and economy.

"It looks kind of hard to criticize Trump's economic success," the Yale University economics professor told CNBC on shortly after the government released its September employment report on Friday.

The report found that the jobless rate dropped to 3.5%, a level last seen in December 1969.

The number certainly can boost confidence, Shiller said, but "the unemployment rate is a fuzzy number."

"I don't know if it is entirely comparable to 50 years ago. We have different thoughts about that now," said Shiller, who won the Nobel Prize in economics in 2013 for his work on asset prices and inefficient markets.