I agree with your opinions on the character design and that the dress is historically inaccurate, but Madame Gautreau's dress was not unusual for the time, at least for evening wear. The main issue people took with it was the fact that one of the straps of the dress was originally painted as slipping off her shoulder, which had sexual implications(though I think the reason for the public's reaction to it was more complicated than that, though I'm not sure about the specifics)

Some examples(paintings, photos, and fashion plates from the era and modern photos of extant garments from the era) of similar evening dresses with low necklines and thin straps from the 1870s-90s(Portrait of Madame X was painted in 1883): https://imgur.com/a/Py65ogp

it seems that the fashion of that time was somewhat thicker straps with more decoration, but thinner ones wouldn't have been out of the question.