The slicked back bit isn't the issue, it's the loose bit in the back. Like what they used to say about mullets "business in the front and a party in the back."
If she's gonna do the slicked back look, she should've fully committed and done the sleek ponytail (like she wore in that gorgeous cutout black Pucci dress in Cannes a couple years ago). That sleek hair would've complimented the dress. Though its not my fav dress either way. But I think in general it's really the hair that's bringing this look down.