That's about right, though I think the "mon" at the end makes it almost sarcastic, as in "It's not as if they are panties, are they?" EDICT gives as definition for もん - "used to create a rhetorical question indicating that the speaker actually believes the opposite is true", though it's rather idiomatic and EDICT often fails with idiomatic expressions sometimes.
I'm going to say that in this case, 'mon' is like an added word to help convince you because you obviously believe otherwise (you, as in the person she's talking to). This is a little backwards from what Shinjidude posted, but it's how I've perceived this word to work.
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