In 1974 and 1976, the Gallup Poll asked "More generally, how much trust and confidence do you have in the American people as a whole when it comes to making judgments under our democratic system about the issues facing our country--a great deal, a fair amount, not very much, or none at all?" This question didn't appear again until 2001, but since then it's been asked fairly often, mostly by Gallup but also by the Monmouth University Polling Institute. The averages, with higher values indicating more confidence:
It was lower in 2001 than in the 1970s, then rebounded for a few years, maybe as part of the general feeling of solidarity after 9/11 (the 2001 survey was taken just a few days before), then declined. So there was definitely a downward trend. But there was a sharper decline between 2010 and 2016, and then it stayed low before rebounding in the latest survey (Jan 21-24, 2021).
A question about "trust and confidence in the wisdom of the American people" showed a similar pattern of gradual decline and then sharper decline during the Obama administration. This is different from the decline in confidence in institutions, which has been pretty steady.
[Data from the Roper Center for Public Opinion Research]