Charisma is essential for con men. It's why "con" is short for "confidence"; in order to get the mark to do what you want, you first have to gain their confidence. That's where charisma comes in.
Fortunately, Trump has been getting increasingly bizarre and confused lately, and rather than try and be more charismatic as he drops in the polls, he's doubled down on anger and hate. While that may increase his base's adoration, it turns off moderates and swing voters.