The guardian article about this says it's not the first fictional character used:
"The choice of Wonder Woman was not the first time a fictional honorary ambassador was chosen to represent the UN. Previously, Winnie the Pooh was an ambassador for International Day of Friendship, and a character from Angry Birds was chosen for International Day of Happiness, according to NPR. Tinker Bell was made “honorary ambassador of green” in 2009."
But still, women and girls need real role models. How can they aspire to be a fictional character who only achieves what she does because she has super powers?