Have you looked at the whole issue? Every other woman at the party is dressed in a "backless" dress with plenty of cleavage and an equal amount of visible skin. (Except Babs. Hers is only "backless".) So however you happen to judge Helena's appearance, she looks like all the other women (12 pictured in the story) at the party.
And like Chipsnopotatoes points out, the slit only shows if she raises up her leg. It's a very voluminous skirt, with lots of fabric.