I'm not sure Helena's outfit in this setting would make her come off as promiscuous, actually. As she mentions herself, she's not wearing a trampy outfit, she's wearing an expensive designer dress--just one made for a woman with the body to show it off. One that happens to have a low neckline, but that's not totally unusual. Given that she's young and in excellent shape I don't think her outfit would necessarily read as trampy--it would say something about her personality in that she would choose a dress with that plunging a neckline where others wouldn't choose that style but it probably wouldn't read "slut" so much as "sexy." The front is cut a lot like Selina's bathing suit.
Clearly the artist designed the dress to project that from every angle so when she's got her leg up there's a slit and when she's turned around it's dipping in the back, but when I saw her in the art she didn't look trampy to me. She is dressed appropriately, as she reminds Dick. She doesn't stand out that much.