Trump's behaviour cannot simply be reduced to his background in reality television. Firstly, when he was in The Apprentice, he was not a contestant, he was a judge.
The judge does not have to play an erratic and impulsive role to boost ratings. The judge is cool and collected and behaves much more like a viewer.
Simon Cowell and Gorden Ramsey do not fight, throw temper tantrums and behave erratically.
Like Ramsey and Cowell, Trump's character evaluated contestants' performances and had the final say over who would leave the show. He needed to possess strong analytical skills and make rational decisions. None of these skills he appears to possess now in office.
Also, his behaviour goes far beyond simply erratic, impulsive arrogance. In Michael Wolff's book, he described a president who would often repeat himself, couldn't follow a simple stream of thought and craved public adoration and popularity. These are not the signs of a TV personality in the White House, they are signs of significant cognitive decline and mental illness.