grammar confuse: there's a senstence of a song confuse me ----i'm no beauty queen ,why is i'm not beauty queen

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no = not a
I'm no beauty queen = I'm not a beauty queen
You MUST use "a" here.

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Maybe there are prettier girls around ,I am not sure :P

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Eliot is right.  

But, it is slang, so people use this in songs or when we talk to each other.  

So, just an example with this specific sentence.  You are on the phone with your friend and she says:

Friend:  I am only friends with pretty people.  All my friends are beautiful.
You:  Well, I'm no beauty queen.

You are basically saying you are not a beauty queen, or, you are not pretty.