Well he is actually very successful given that 1/3 of his business endeavors met with success. That looks like failure to the outside world, but in the business world, that's above average. Businesses fail for a variety reasons and not simply because of mismanagement.
We had a new restaurant that opened in my town. It was a Scottish Highlands themed restaurant that prided itself on only using imported Scottish beef. But then came the mad cow disease scare in Great Britain and the restaurant could no longer import the beef and quickly went under and closed up.
In my town, we had a major real estate developer, one of the richest men in our part of the state. He opened businesses that didn't take off. Sometimes you have an idea that simply doesn't catch fire with the public and thus, it fails. But he had a mixture of success and failure but was still considered a great businessman.
Starting a business is hard and it involves a lot of risk and sometimes it pays off and sometimes it doesn't. And in the case of businessmen like Trump and other billionaires, they have a lot of irons in the fire and they don't personally manage/run every business they start. Sometimes, those in management make bad decisions and end up jeopardizing the businesses. So you cannot say that Trump's failures are the result of being bad at running things.
What a lot of people don't talk about is how often many, many people in business have successes and failures and they don't talk about all of the variables that go into being successful business person. Some ventures strike gold, and some don't. We live in a free market economy and business success is often determined by the market. If someone has a better idea, or improves on an existing idea, they are successful and those who were operating on the old method can end up losing out and are forced to close down if they can't adapt.
In business there is equality of opportunity, but not equality of outcome. If EVERY business Trump started was a blazing success, it would be suspect because people in business often fail more than they succeed.