Yes. In that she was very different from her siblings. I think that was what Mimi said about "her glamour gone".
Let's get the timeline straight, shall we? I am quite annoyed by many writers referring to that line to prove her "rebelliousness" and "unhappiness". Mimi visited in 1776. Other than that remark, which I don't find very credible at all, we have no other "guide" as to her looks,glamour, dressing style, etc. except for her portraits done 1776 onwards and that remark by an English traveler in 1783. Her portrait by Alexander Roslin looks good, arguably better than Mimi's by the same artist, and that was in 1778. We have at least two portraits seemingly done in her late 30s. Then her older portraits by Zoffany (1791) and Veiera (1795) and a few unnamed ones seemingly done by the 1790s. Certainly, by her mid/late 30s her facial features turned sharp, just like her sister Maria Anna's, but it seemed like a genetic thing because they shared similar physical problems, not because she was "miserable" (as Mimi seemed to imply) or out of rebellion/wildness or whatever else.
How is Maria Amalia different from her siblings then? I very much agree that she wasn't a fashion plate and could have dressed better. On the other hand, I think it is to her credit that she didn't, at least no one can accuse her of wasting money on such things.
Marie Antoinette spent a lot on clothes and accessories but did that keep her young and good-looking (not that she was such a beauty in the first place) and we know that she was fat and looking older than her years by the mid-late 1780s. I have never read that Maria Carolina was such a fashion plate although she also patronised Rose Bertin and Leonard (Marie Antoinette's dressmaker and hairdresser); she wasn't also a beauty. I have no idea whether Mimi was fashionable but judging from a portrait (or was it an engraving?) I saw of her in Brussels, she was very fat and rather ugly by the early 1790s.