The importance of the painting lies in the context. At the time, 1863, first exhibited at the salon in 1865, paintings of Venus coyly glancing at the viewer, neoclassical figures, composed landscapes of the "ancient" where popular and acceptable. The only accepted female sexuality in art lies in Venus paintings and commissioned portraits of someone's wife or lover, in a neoclassical style, and most likely will be "airbrushed", if you will, to perfect their figure, and will probably never ever be seen by anyone but the artist and the patron.
Now, Manet, in a time period where it is considered downright un-lady like for a woman to show a bare ankle in public, displays a young girl as a courtesan (prostitute), gazing directly at the viewer, in the exact same pose as the very famous and revered painting called the Venus of Urbino or other famous Venus paintings. Another shocking detail, he painted her as is, without perfecting her figure.
So as society saw it, he painted an ugly un-lady like child courtesan glaring at the viewer as she attempts to put on the air of a classic art subject. On top of that, it was exhibited in the Salon! The pre-eminant institution of "good" art at the time. If you were worth your salt in France in 1865, you had been accepted into the salon...so why was this "trash" in there?
In essence, Manet is acting like the Banksy of early modernism. He uses an ironic juxtaposition of a modern figure to a classic one to make a point about modern life.
Hope this helps!