She knows how crazy Donald Trump can be and now it pisses him off because not only has she written a book from her professional point of view on him— showing how narcissistic he can be, she's very open to talking about it.
Donald projected his character saying “She's a mess on twitter.” He usually does this kind of stuff. When he feels he's caught up he lashes out with the worse comment he can find.
I just love how Mary Trump is calm and collected. Kind of makes her look like the black sheep of her family. Lol.