The effectiveness of NATO is entirely based on the threat of retaliation from all member countries, in case one of them is attacked, being credible.
While asking all member countries to pay their fair share may be reasonable making statements which reduce the credibility of the threat is massively irresponsible.
And since you bring up the Art of the Deal you may "enjoy" this article by Tony Schwartz, the ghost writer of said book. http://www.newyorker.com/magazine/2016/07/25/donald-trumps-g...