I think it makes total sense that she'd wear a flashy and more feminine take on a normal magicians outfit. She's styling herself off after her father after all while still projecting herself as a fancy female magician. She's not dressing to be objectified but to come off as flashy and sexy. I think there's plenty of whimsy in her costume.
I see no whimsy. What the heck even is whimsy in clothing? I googled it, and saw more flowy-things, which is about the only thing I could think of myself.

While I don't loathe the fishnets, I think they work better as a stage costume than a hero costume, and I think she is often indisputably, heavily sexualized and objectified. So very many arched back/butt-and-breasts-stuck-out panels. For almost all the women really (seriously, compare Zatanna's original costume - certainly less bottom coverage, but more importantly than that, the entire way she's drawn is simply far more sexual/cheesecake in modern times). And many of them, even in casual or battle situations, heavily drawn to be viewed primary as objects of sexual attraction. Zatanna definitely hits that more heavily than many. Huge breasts in certain eras/artists. And I much prefer a ruffled or pleated shirt - so she doesn't have a painted on look. Oh the fade-off-clothes-to-nightgown in issue 2 - that's pretty "male gaze" oriented to me. As were some of the fighting bits. Hardly the only character to go through such (oh, the intended-to-be-sexy-looking torture bits in the 1980s NTT), but still frustrating. The back of Zatanna's outfit also shrinks on occasion to show us more butt at times during the the Dini times, too.

While the women so frequently having skimpier outfits (and Zatana's becomes more and less skimpy at times) and thigh-high boots is annoying to me, it isn't nearly as annoying at they way they are drawn. While people talk about big muscles on men in comparison, at least they have a purpose in a physically-fighting hero in a way women's extreme figures don't (and, honestly, I think they're meant to appeal to men more than women, too - it's the demographic most comics have). More than that, though, it's the poses and movements. Butt-and-boob shots (whether when casually standing around or in battle), stances that make no sense, "posing" for the "camera" angles, etc. It certainly seems to me that special effort is taken in to draw women in a way that draws they minds to sexual attractiveness much moreso than in done with male characters. So art style is as important as actual costume. Caveat: though not usually a problem with Zatanna (she usually has a shirt), I do wish women would not be drawn in outfits that would not keep their breasts inside during a battle (where people get thrown around, etc.).