Symbolism, Imagery, Allegory
Becky puts on many different costumes throughout the course of the novel. Some are completely literal, like the sexy toga she busts out for her star-making turn in Lord Steyne's charade. There's a lot going on in that scene. First, Becky is revealed to be an amazing actress, which is great for the stage but not so much for honest, real-world relationships. Second, she makes for a really convincing Clytemnestra, a woman who murdered her husband, which doesn't bode well for Rawdon or, later, Jos. And third, this triumph is the beginning of the end for her, since the thing that gets her to the top of the social heap – her appeal to men – is the thing that will be her downfall.
Some costumes are metaphorical, like when the narrator describes Becky as a siren: the part above the water is beautiful and can sing amazingly, but there's a horrible cannibalistic monster underneath. Clearly Becky doesn't literally eat people, but she does use her considerable charms (a.k.a. hotness) to lure men to her, extract as much from them as she can, then casually toss them aside. This comparison is also a way to point a finger at the audience. After all, we've been sitting there the whole time totally on Team Becky, not giving a second thought to how thoroughly immoral she actually is.
And some costumes are part literal, part metaphorical. For instance, the white dresses Becky wears on her honeymoon. White equals virginal and pure. And Becky? Not so much. But that's what the world expects to see, so that's what she wears. Or take the little white shirt she pulls out of her sewing box and works on every time she needs to look all feminine and maternal – a little shirt that the narrator tells us has been way too small for Rawdon Jr. for quite some time. She uses it most when helping Pitt with his Parliamentary career, which makes sense, because back in the day, women weren't supposed to be involved with men-only things like politics. What better way to be all "I'm just a little housewife listening to you talk about things I don't really understand" than to do some embroidery.