Although Banksy already awarded himself an Oscar (complete with stormtroopers to protect it!), the Academy did not officially give him one.
The little girl is the 15 month old daughter of a producer of The King's Speech who apparently dropped and broke his Oscar. What comment Banksy is trying to make is up to us to figure out -- is he pouting about the loss? Making a mockery of institutional awards? Who knows/particularly cares?
My current favorite hypothesis is that Banksy, who has managed to somehow maintain the anonymity necessary to be a tagger despite international fame and being the subject of an Oscar-nominated documentary, is actually that little girl and this is how she has chosen to reveal her identity to the world...