She writes and talks about Trump because she feels that the insight that she has on the fucked up dynamic of the Trump family is useful in understanding the mindset of one of the most powerful men in the world. People care about what she says, and thus give her a platform, because they agree that her analysis is useful and interesting.

In terms of her agenda, if I were her, saddled with the curse of that name and the toxic family that comes with it, I would feel it my duty to do everything I could to criticise Trump, especially given that her name means that her words would carry weight even if her perspective wasn't especially interesting (I do find her work interesting — she doesn't just coast off of the name, but also draws on her experience as someone with a PhD in psychology). Hell, even outside of that hypothetical, I already do consider it my duty to oppose Trump however I can; it's just that that amounts to very little given that I'm a Brit with no political power). Trump is such a repugnant human that surely we don't need to grasp for some nefarious underlying agenda to explain why she'd criticise him.