No matter how much bragging he does or how many lies he tells, Donald Trump has not been able to get anywhere near a majority of Americans approving of the job he is doing. In fact, the public disapproval has been remarkably stable (moving up and down a few points, but always significantly negative).
I believe this is because a majority of the American public simply does not trust Trump. They consider him to be dishonest and untrustworthy.
Above are the results of four different respected polls on the question of Trump's honesty. All of them show that a significant portion of the public considers his dishonest (from between 19 and 21 points in all the four polls).
The Politico / Morning Consult Poll was done between July 19th and 23rd of a national sample of 1,996 registered voters, with a 2 point margin of error.
The Economist / YouGov Poll was done between July 22nd and 24th of a national sample of 1,500 adults, with a 2.8 point margin of error.
The Quinnipiac University Poll was done between June 27th and July 1st of a national sample of 1,020 voters, with a 3.7 point margin of error.
The Public Policy Polling Survey was done between June 8th and 10th of a national sample of 679 registered voters, with a 3.8 point margin of error.