According to Longman Dictionary the droll comeback...
and I’m the Queen of Sheba
(humor) used as a reply when someone claims that they are famous or that they have done something impressive, but you do not believe them
‘I’d like to reserve a table for tonight – this is Demi Moore.’
‘Oh yeah, and I’m the Queen of Sheba.’
A movie poster of The Queen of Sheba, an Italian drama/romance movie released in 1952
Amazingly, Phrase Finder has no information on this pithy rejoinder.
I believe that the catchphrase is a snowclone of “I'm the Queen of England” or "I'm the King of Siam" but which came first? And why is it the Queen of Sheba and not some other ancient or mythological kingdom?
I have a sinking feeling that it is terribly old-fashioned nowadays. What would be its modern-day counterpart?