Clarification: Under the controlled conditions under which the press slavishly serve while reporting on Hillary, yes, she is able to fake serious and "presidential" better. Of course in real life she's cruder, ruder, and nastier than the worst parody of Donald Trump. But to find that out a voter would have to dig past the media shield. That code of silencio o muerta will only continue and become an even stricter through November.
But my main point was that the other Republican candidates are polling better against her now because more voters see them as apples to apple options. Trump is as much or perhaps even more polarizing than Hillary. His fans can't see it and there's no convincing them. But this is my honest opinion.