That dumb ass nail polish leitmotiff

I was wondering about that, i.e., when Rosalyn keeps going on about how the nail polish smells sweet but also like there's something rotten in it. Is that a thing? She says something similar, though about perfume, to Edith in the bathroom scene: "It's like that perfume that you love, that you can't stop smelling even when there's something sour in it."

Like, what is up with nail polish and/or perfume smelling good but rotten or sour too?

I think it's supposed to be some weak analogy for the theme of deception in the film. People are not who they seem, constantly conning each other, etc.

It's also a very good character trait. She is obsessed with this polish, her mind is one-track. She has no ability to think outside her own world - etnocentric. So everything, even when talking to "outsiders" she brings it back to this silly and banal observation that the polish is both sweet and has something rotten to it.
And that "he can't get enough of it. Can't get enough."
It's a great observation on her shut-in existence and how, in the end, she ends up running away with the first person she meets.

Seems like an IMPROV line which the director decided to run with. STOOPID