I think she's petrified beecause she's realised that compared with strong first ladies like Hillary Clinton, Michelle Obama and even Nancy Regan, she's an intellectual lightweight. She isn't educated, doesn't claim to be educated, isn't a lawyer, activist or anything else. She doesn't have English as a first language. She is smart enough however to realise her failings and knows she doesn't measure up. That's why the native English speaking and much more articulate Ivanka steps in.
However, in that situation, the Donald should allow her to take the "stand still, smile and nod" role. Say nothing. Look supportive. But it's all about him and not about her, isn't it?