That's all a matter of values. By hiring her, Dropbox gives the impression that they are endorsing her and what she did in the past, or that "her past does not matter". Which is a very sad call to make when you respect human life.
I understand why people believe this, but strictly speaking, it isn't true. Employing someone isn't an endorsement for all their positions, and even more to the point, employing someone who was in the past affiliated with an organization isn't an endorsement of the whole organization.
Again: I wouldn't hire her, and I'm more confused by her presence on the board than anything else.
I think Rice is towards the bottom of a list of architects of the Iraq War, for what it's worth; only Powell is further removed from it. It probably goes Cheney, Rumsfeld, Bush, {Wolfowitz, Feith, Bolton, &c}, a bunch of other people, and then Rice and Powell. She's a critic --- not very credibly, but a critic nonetheless --- of the actual execution of the war.
I wouldn't hire her, but my reaction to her involvement with Dropbox is less visceral than, I don't know, puzzlement?