Not really, at least not one that's consistently followed. I mean, as Gryphon noted in New Tricks, the configuration of the Striker Unit would make it difficult to wear pants in an operational capacity, but they don't wear them off-duty either, and in practice the phenomenon extends beyond Striker-using witches. For instance, Yoshika's mother, a witch doctor,* doesn't wear trousers or a skirt either (though her doctor's coat does reach to her knees). (Her mother wears hakama, but then, she's an old lady. That is presumably her prerogative.)

It might thus be a general witch thing, but there again, in various scenes we also see hospital nurses (magical status unclear) and Yoshika's childhood friend Micchan (who specifically does not have any magical ability) similarly un-bedecked. So if you go by that and don't assume that it's an animators' oversight, it appears to just be customary in the setting that young women don't wear lower-body garments other than underpants, witches or not.

(There's also the suggestion that it's to provide clearance for their tails, but then Fusō Navy witches, e.g., Mio and Yoshika, wear one-piece swimsuits with a slot at the back for the purpose; so that doesn't stand up to scrutiny either. As it were.)

For OWaW purposes, I'm proceeding from the assumption that it is in fact a witch custom and not something muggle women do. This is contradicted by the canon, as I've noted, but I think it makes more internal sense.

--G.
* One never really gets tired of this.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
Eyrie Productions, Unlimited http://www.eyrie-productions.com/
zgryphon at that email service Google has
Ceterum censeo Carthaginem esse delendam.