I have an incomplete interpretation. I think that this song is about a young woman who grew up very rich and as such never needed to learn anything about the real world. Being born with a silver spoon in ones mouth is a common expression in the US which means a person who was born into a very rich existence.

Coming in through the bathroom window means that she either did not know the normal way of doing things, or she just didn't care. Projected by a silver spoon meant that she, like all of us, was a result of her existence. In her case, she was wealthy and perhaps isolated from others because of her wealth.

Anyhow...

Sucks her thumb indicates that she is childish and doesn't really know how to handle herself in an adult world. Going back to the idea of the silver spoon, that is, she grew up rich and not needing to ever learn anything for herself. Who needs education or even social skills when you have the millions of dollars to hire people to do everything for you?

The part about her always being a dancer, 15 clubs a day. Well, obviously there is not enough time in the day for anyone to dance at 15 clubs as a job. This part probably means that she spent all her time in idle activities, which nothing much to live for. Once again, going back to the idea that she grew up without any need to learn or push herself towards any goals. Still by the end of the song she has become the love interest of the person singing and is trying to help him with needs. In this effect she finally does have a real reason for living and is not just letting life wash over her.

I think this is one of many Beatles songs poking fun at the extremely wealthy in society.

I'm sure someone else can add to my interpretation and improve it.

Renee