This seems to be the conventional wisdom on both sides of the aisle, and it is has been pressed relentlessly by the, um, ... press. But it is a shallow conclusion based on shallow evidence. Trump can be baited into saying impolitic things. In fact, he doesn't even have to be baited. He says impolitic things unprompted all the time. He has the political equivalent of Tourette's syndrome. But is that a substantive failing? Perhaps it is in a President. I've always felt that was George W. Bush's greatest flaw. His inarticulateness made it difficult for him to justify his ideology and his decisions. If he had had the eloquence of Tony Blair, the Iraq War might have been seen as a triumph rather than a disaster.
On the other hand, eloquence is clearly of secondary importance. Tourette's syndrome might be a negative quality for a President to have, but it is hardly as bad as the press, and certain opportunists on the GOP side, are making it out to be. For all of the fear and anger and hatred that has been directed at Trump, almost all of it is for stuff that he has said (usually imprecisely and with little or no thought) rather than for anything he has actually done (or will ever do, even if he were elected).
I can't help but be fascinated by this massive construction project of the political establishment, including the news media, of turning molehills into mountains. Of course, with respect to the news media, they do this every four years with Republican candidates. In this respect, Trump is probably an average Republican nominee. He is worse than the last one, but better than the next one.