
I think it was kind of negative tho, cause MC said 'I picked the most plain one, I'm sorry if I stepped out of line' and then pretends to cry. And mc says she can't say no because it's an old dress that doesn't fit her/she doesn't wear. The maids judge in the end cause why aren't you letting her wear that when you have all the other ones? Like oh she can't even share an old, white dress . Is kinda funny. All she had to do was say She wanted to keep it safe because it's a gift from her fiance. Then they'd understand

It does seem it was portrayed that way in the webtoon, they omitted a few thoughts of Aria and her observations. In the novel she remarks (to herself) how the maids (Mielle's) are still hostile towards her, throwing her dirty looks since they see her as a brat who want to harm their beautiful and kind lady.
Also, the dress was described as simple in comparison to her other dresses. I doubt it is any less pricy that the others. It never occurred to me while reading the novel that they meant it in a negative way. Aria was fucking with Mielle, phrasing her words to advantage to seem more humble, and making Mielle look bad in the process. She is manipulating the situation and trying to gain pity from the maids and the people around. They judge her because she said "you can choose whatever you like" or something along those lines, it is considered a bit shameful to go back on your words, more so since Mielle said it to seem generous now saying no to a white simple dress would be a hard hit on her image.
The maids would never dare to verbally question their master (Mielle), their whisper behind her back does not make sense. I am guessing to emphasise the pressure Mielle had in the moment to keep her good image they did it like that.
In the novel it hard to write so that every side character gets a bit to talk, or at least it was not deemed necessary because it would contradict with the maids' characters as the faithful servant.
I just tried to make sense of it in a way that didn't contradict the novel.
Sorry for the long text..

Oh no I totally get you (in just the webtoon, how the artist portrayed it) to me I saw the maid's reaction as negative towards the sister. Mc really knew how to trap her step sister to make sure she didn't go back on her word. Idk is like the maids will always gossip so you have to pretend to be the angelic little girl so they don't spread rumors about you. I think this will be a more prevalent subject in the upcoming chapters (I haven't read the novel so I don't know the comparison )
what's wrong with the white dress ? why were the maids saying 'oh she has a white dress like that' as if it was a promiscuous dress or sth