WASHINGTON—A strong economy and a polarizing president have left Americans with mixed feelings about the nation’s course. A majority say 2018 was a good, or at least average, year for the country, but also that the U.S. is on the wrong track.

According to a new Wall Street Journal/NBC News poll, one-third of Americans say the economy will get worse in 2019, the highest level of wariness in five years, the poll found.

The...