Don't forget that being young, female and attractive is a
complete disqualification from having opinions, expressing them, or taking part in any activity more serious than painting your nails.
As should be obvious to anyone who's been exposed to male sportsmen, for example, being trotted out as spokesmen for everything from selling cars to endorsing haemorrhoid cream, the whole point of having celebrity ambassadors is to persuade people to pay attention. Using your celebrity to draw attention to causes you believe to be just and vital is at
least as worthy as using it to sell golf clubs or lipstick, and being an actor by profession is no bar to having a brain or a conscience.
As for the United Nations, it is as effective as the nation-states that make it up want it to be. Typically people who whine about how ineffective the UN is would be the first to complain if the UN could act effectively to control the behaviour of
their governments. From Americans, for example, the complaint simply boils down to:
"Waaa! The UN doesn't do what America wants it to!".