MEXICO CITY—An unlikely honeymoon between Wall Street and Mexican President-elect Andrés Manuel López Obrador, a leftist nationalist, is over before he even takes office on Dec. 1.

In the months that followed Mr. López Obrador’s July election win, the peso and Mexican stocks rallied as foreign investors appeared willing to give the former Mexico City mayor the benefit of the doubt. He had made a pragmatic- sounding victory speech, vowed to keep Mexico’s finances in check, and appointed a businessman as chief of staff and...