...hmmm... it may not be against the code of ethics and sure psychiatrists can make an assessment based on what they see and form a 'clinical' opinion. However, without psychiatrists actually sitting down and talking to him at length to evaluate him, I tend to take the diagnosis with a grain of salt.
I do feel Trump is a narcissist based on definition. Many past presidents and world leaders have displayed narcissism but that doesn't necessarily mean that their leadership was bad for the country.
By the definition, I don't view Trump as 'malignant narcissist', he certainly isn't anti-social or paranoid, nor have an absence of conscience; quite the opposite.
Kernberg described malignant narcissism as a syndrome characterized by a narcissistic personality disorder (NPD), antisocial features, paranoid traits, and egosyntonic aggression. Other symptoms may include an absence of conscience, a psychological need for power, and a sense of importance (grandiosity).