Every woman I've asked about this has replied that going topless would be more trouble than it's worth; there'd be so much staring, and so many negative comments, that the "freedom" of doing it would be worth less than just wearing a top.
Now here's a "Talk of the Town" section from The New Yorker, February 18, 1928:
The progress of the nicotinization of Women has been interesting to watch. Cigarette manufacturers have admitted openly that women smoke; the railroads have provided ladies' smoking-rooms on the more venturesome trains; and even the Old Lady from Dubuque has learned, through hearsay, that smoking occurs in both sexes. It strikes us that the only party not completely emancipated is Woman herself, for we have rarely seen a woman smoke on the street, the reason being--so our cigarette-borrowing lady companions advise us--that it attracts undue attention and the satisfaction isn't worth the embarrassment. To be exact, the only times women smoke in the street are on the sidewalk in front of playhouses during intermission, and the three or four steps they take getting into or out of a cab. We, who have supplied literally tens of thousands of cigarettes to ravenously beautiful smokers, will never be convinced of the reality of their fag-life until we see one of these ladies enjoying a brief siesta on a bench in Madison Square, non-chalantly smoking.I'm surprised that women had problems smoking in the late '20s. I'm pretty sure that women were publically smoking by the mid-'30s, though there was probably still some stigma. Nowadays there doesn't seem to be any smoking stigma at all, other than the one that covers both genders.
Incidentally, any lady desirous of learning to smoke in the street should know that the finest street in which to practice is the old half-abandoned mall right behind the Library, where a lady may pace and puff, in what travel literature might describe as Old World Splendor.
I suppose that going topless is quite a bit different from smoking, in that it's something disapproved of even in men, and that for women it would be considered an outright sexual activity (which smoking probably wasn't, at least not overtly).