They are continually saying this so that it will stick in people’s minds as a knee jerk response - Trump is mentally ill. Can they define what they mean? What are they talking about?
Yes, and they say how uncouth he is and how he hates women and minorities and many other things.
They think we’re stupid.
Trump is an unusual person but does not have a personality disorder. Indeed, Trump’s fluidity and his successful innovations and departures from accepted practices and policies are contrary to the inflexibility and rigidity that are aspects of personality disorders. And I know two people who had dealings with Trump in years past, and both of them found him pleasant and easy to deal with.