For me, it's at least: Does this fit the character? And are you Jessica Rabbit-ing her? As in, it's not a bad look, just drawn that way.
Emma's personality-type is one where I can totally buy her choosing a revealing costume. She leans into her sex appeak, and honestly strikes me as someone who would would rather die than dress practically. Fashion over function. Also, with the inclusion of her diamond form, she's a bit of a tank and can actually take hits! See also why Colossus can wear his booty shorts. Contrast that with
Huntress belly window, which doesn't fit the character and is honestly a very terrible strategic choice for someone squishy. Likewise, Sue Storm's boob window '4' also didn't fit the character at all. Even though she can create shields and protection, she's just not the personality type to run around in battle belly and boobs-bared. Then again, I also hate Jean's go-go dress, because it doesn't strike me as something a woman in her late-twenties would choose to wear -- but that also goes with her calling herself Marvel Girl.
As for the 'not bad, just drawn that way" -- I feel like that's something a lot of female characters face. Take Carol Danvers' iconic
Ms. Marvel costume for example. It's honestly one of my favorite comic costumes. It's just so aesthetically pleasing to look at. Her power set allows her to pull it off. I actually hate her current costume because I feel like it lacks personality. The swimsuit look is fine in and of itself, and evocative of what many gymnasts wear. (And yes, I do know some are pushing back against the leotards.) However, too often it was drawn with heels and and basically looking like a thong cut that it *felt* trashy.