And this particular costume design doesn't say to me, "We're embracing our bodies!", it says "Hey neckbeards, lookit the tits!".

Exactly. Why would anyone seriously argue that complaining about female characters in extremely revealing clothing is a slut-shaming act? It should be fairly obvious that the use of female characters for gratuitous sex appeal is sexist, not empowering. The OP's grievance wasn't specific to female characters' costumes—they made that as explicit as possible—but if you've ever seen a male character whose outfit was designed to designate him as a sex object, please let me know.