What does 'How you doing?' by Joey mean?

The expression How you doin'? is a modification of the English How are you doing? which is another way of saying "How are you?".

This phrase seems to be used by stereotypical mobsters in popular culture, who usually were of foreign descent and so would pronounce expressions with an accent. For example, Tony Soprano in the third season (the 5th quote in Another Toothpick):

Tony Soprano: How you doin'? I got new shoes. Soles are a little heavy

Joey does the same (probably a reference to his foreign descent) but in his case, the sentence is a pick-up line, a conversation starter for his die hard promiscuity with women.