Except that he has made incredibly stupid unforced mistakes?
Even Steve Bannon has said the firing of Comey was one of the largest mistakes in modern Presidential history. The fact that he brings issues back into the media that would otherwise have died, over and over again, whining about things like a petulant child.
He had the Justice dept say that Comey was fired because of Rod Rosenstein’s memo, then the next day goes on NBC and admits it was because he wanted to get the Russia investigation over with.
He revealed highly classified information to the Russian foreign minister and ambassador in a White House meeting, and while that is his prerogative as President, it’s still stupid and careless (unless it was part of a larger plan, which I’d bet a months pay it was not).
Fake Time magazine covers (while not a political mistake, it is a public relations mistake, a bizarre one at that).
Anthony Scaramucci (enough said)
Bazaar lines about crowd sizes and convincing people that his election win was “historic” and the largest win in (insert timeframe)
After promising throngs of Boy Scouts gathered at the group’s National Scout Jamboree that he would not delve into politics, Trump went on a 35-minute rant dissing his rival Clinton.
These sorts of mistakes aren’t because he isn’t a politician, they are because he is a habitual liar and obviously extremely insecure about who he is.