If she'd worn a posh frock, I'd bet my bottom dollar the press would have been all over her for being 'overweight' and 'mutton dressed as lamb'.
And yes, she probably wouldn't have turned up in couture because she almost certainly doesn't earn anything like those in more high-profile positions; and if she'd worn a 'normal' dress from a shop, again it would have been pored over and critiqued to high heaven (I can imagine words like 'cheap' and 'high-street' – as a pejorative –being bandied about).
Those arseholes like the overrated Inarritu not clapping and sitting there with cat's-bum faces ought to be ashamed of themselves.