A Washington Post–ABC News poll taken the last week of May 2020 asked Americans, “Do you think Trump has the mental sharpness it takes to serve effectively as president?” Fifty-two percent of respondents said no, with only 46 percent saying yes.
One might see this solid majority response as the weary, off-the-cuff judgment of an American public worn down by Trump’s barrage of outlandish claims about coronavirus treatments, or fantasized accounts of legions of violent antifa leaders orchestrating the present nationwide protests over the police killing of George Floyd. (Indeed, since that poll’s release, it was reported that Trump mistakenly tried to register to vote in his newly adopted home state of Florida using an out-of-state address.) But in truth, this was far from the first poll to find that a substantial number of Americans see Trump as not very bright. An Economist/YouGov poll in 2019 asked, “Compared to other presidents since World War II, would you say that Trump is more or less intelligent?” Forty-seven percent said that he is less intelligent, 22 percent said he has about the same intelligence, and just 21 percent thought he is more intelligent.
It appears that Trump’s performance in office has had a negative effect on perceptions of his mental acuity. The Quinnipiac poll tracked perceptions of Trump’s mental sharpness from 2016 through 2018, asking, “Would you say that Donald Trump is intelligent, or not?” When first asked in November 2016, 74 percent of people said yes, and only 21 percent said no. A year later, however, those answering in the affirmative had fallen to 55 percent, while those in the negative camp rose to 41 percent. Subsequent polls found roughly the same ratio.
https://newrepublic.com/article/158069/donald-trump-not-smart-polls