Being a leader, believe it or not. The crown on her head, (as well as her flank) are symbols of leadership. She leads, Silver follows like the tool she is. Although she says her talent is being a princess, in some ways she's wrong, but in others she's right.
After Filthy Rich dies, whose the proverbial King of the Castle, Diamond Tiara will inherit everything he ever had, essentially boosting her title of "Princess" to that of "Queen".
The cutie mark is not always the special talent, it could also be what the pony likes most. Applejack did get her cutie mark when she realized she belongs to her family, not when she was bucking apples.
Silver Spoon's cutie mark resembles a spoon because the thing she loves most is herself.
Well thinking it is a silver spoon, and how she hangs around dimond tiara (we know she is super rich) she must be as well from a rich family and her manners, i guess are just amazing when she tries, you know how they say "he was born with a silver spoon in his mouth" well that basiclly means someone is from a wealthy family. Well that is only what i got from her cutie mark, but again i am guessing i am wrong. Who knows, maybe they will explain it in 4th season.