Of course we must remember Mimi was written very subjectively on her sister. As you say Mimi had grown fat, but I think she still dresses luxuriously with appropriate amounts of jewelry. I think you wrote that Maria Amalia had in some point to sell her jewels. So dressing shabbily might be Mimi's "requirement" that her wayward sister had finally lost it.
I'm just trying to set the context/timeline straight. Context is as important as facts. Yes, Maria Amalia dressed shabbily for a royal but it seems that started by the early 1780s, not when Mimi visited. Yes, she had to sell and pawn her diamonds when the pensions from France and Spain stopped for a year (1772-73). She also sold them to pay her personal debts (which is a very good thing rather than dipping her hands into state coffers).
Mimi and her husband Albert managed to "siphon" a lot of money and presents (jewels, paintings, etc.) while Maria Theresa was alive; it was one thing that irked the thrifty Joseph II very much. However, the indulged Mimi didn't even have enough funds to transfer from Pressburg to Brussels after MT died. Leopold had to lend her 200,000 florins for it. If Mimi continued to dress luxuriously with appropriate amounts of jewelry after MT died, perhaps it was because she had more than enough from her mother. But it didn't seem as if she was cash-rich after MT died, if she were she wouldn't have to borrow money from Leopold. Joseph did say that Mimi had a lot of dealings with a banker in Brussels so it was likely that she was trying to make more money to support her lifestyle post-Maria Theresa; Joseph II certainly not indulge her that way.
As you written that Maria Amalia wasn't that much interested in clothes and court balls, so that would explain that why she would look shabby anyway to her critical, proud and sharp tongued sister.
Again, context and timeline. Maria Amalia seemingly became disinterested in clothes and court events later on, not during Mimi's visit.
Also Marie Antoinette may have lost her figure, but not her famous charm and wonderful complexion that even in her worst appearance was able to fascinate people. Hence lies the fatal attraction of her to historians & biographers even after 300 years after her death.
Again, context. yes, Marie Antoinette was very charming but only to selected people and when it suited her. And a wonderful complexion? Not entirely. She had facial marks from smallpox, which was concealed by powder. Maria Theresa fretted about it before her marriage. Her "maid" Rosalie at the prison also saw the marks when MA stood by the window with the strong sunlight. It would make sense that her facial pock marks were noticeable then, she didn't follow her old toilette nor had access to expensive make-up. And she was older as well.
I think MA's "attraction" mainly lies at her ultimate fate (which she didn't deserve) that's why she is much written about, not her selective charm nor her mythical "perfect" complexion.
