it's not"你凑美“，it should be "你臭美",which means someone is very conceited, and maybe selfish.but in china,it is often a banter among friends.

As Lynn mentioned it is "你臭美" and refers to someone showing off their good looks shamelessly

你凑美---你臭美 It's jokinig words, used between good friends. When people feel full of themselves, wanna show off their good things, such as good dress, etc. You may come out with this word"臭美".

你凑美---你臭美 It's words used between good friends,something like joking. People feel full of themselves, wanna show off their good things,especially good faces, good appearance, etc.You would say 臭美.

For example:if you bought a bottle of cola for yourself in a very hot summer.One of your friend said:is it for me? Then you can say:你臭美。 And the example taken by Jessy is another way to use 你臭美。