If Billie Eilish wants to do the "Marilyn" thing, it's obviously for a bit of fun—remember fun? No, neither do I—and for shaking her image up a bit.
She's wearing what SHE wants to wear, that feels good, makes her happy in her own skin and it was obviously great publicity. The only statement I see in this is "don't fence me in."
When women were expected to dress like that, was a different time. (Wasn't it?) What people are doing by putting her down for her choice is viewing her actions in terms of men's desires and seeing her as complicit in her own oppression.
Women are accused of "asking for it" by dressing in what is called a sexualized way. That is victim-blaming and the real oppression: making women responsible for men's desires and always having to consider the male gaze; women as pawns whose every move must in some way be dealing with men, who simply can't be expected to control their unruly urges.
I assume that women dress to please themselves. End of story.
That's my take away as a gay male, but who cares what I think? I'm most keenly interested to hear what women say...
DR