She was also mistaken for a child at the end of the second book, in the village where she got the horseshoes. They ask where her parents are.
She is dismissed often in the first book. Even when she's used to the ways of court she's considered an oddity among them, so no real mention of stunning beauty there. Elend is in love with her, so of course he's going to think she's beautiful. As for grace, we're not talking drop-dead gorgeous. Straff pulled out a younger and (in his opinion, granted) more pretty girl with a wave of his arm.
As for the dress shop, beauty is never mentioned. Grace of motion, however, is, and grace isn't necessarily equated to beauty. In fact Thugs are often said to move with 'the grace of a man burning pewter.' In the beginning of the Hero of Ages she is referred to as five foot tall, while comparing her to the Koloss. And that's after she's
finished growing.
Not, she might have been young and pretty in a dress during the first two books, but not 'hot' until the end. In fact, I can quote from the third book, page 257 of the paperback:
"'Ah, Vin,' Ham said, leaning back and folding his muscular arms, 'when did you go and grow up on me? It seems like just last week you were scrambling about, hiding in corners, wearing the haircut of a boy and the attitude of a mouse.'"
This observation of her in a dress hints that she is in top form, having truly matured in Ham's eyes. You could say that this is invalid because she is never in a dress, but Ham saw Vin when she came back from Straff's camp after intimidating him, and maybe before. He wouldn't be as surprised if he was already aware that she was, as you said, ' absolutely gorgeous.'
Hell, Elend didn't even seek her out while looking for someone attractive. He first met her while trying to annoy his father.