Lady Middleton recalled to mind Lady Bertram for me also. Not only are they both uninformed and indolent, one coddles her dog and the other her children. :) It’s admirable to care for your children but there is a difference between nurturing and spoiling.

In Marianne and Elinor I see sentiments of Jane Austen, herself. She liked children and felt they made great ice-breakers, particularly when new people were being introduced into the mix but I have noticed in her letters that when she spoke of children she generally commented on their behavior as she approved good behavior and disapproved bad behavior. Letter of 1/24/1813: Dame G. is pretty well, and we found her surrounded by her well-behaved, healthy, large-eyed children. Once, the conversation of any children present and the weather had petered out, and with a lack of common acquaintances, the conversation would then likely turn to books and authors, music, plays, travel, any of the general pursuits of the accomplished lady. Lady Middleton, who as a single woman was likely an accomplished woman, (definitely in music) has, due to indolence, abdicated all this, now that she has gotten her man, so lacks the ability to engage in good conversation.

My understanding is that good etiquette required members of the gentry to uphold their share of polite conversation. Austen’s letters indicate that she had no patience for those who did not. I recall a letter where she was visiting some people or vice versa and she complained that they did not contribute to the conversation stating something along the lines of how they just sat and looked at them. In letter 12/27/1808, Our evening party on Thursday produced nothing more remarkable than Miss Murden's coming too, though she had declined it absolutely in the morning, and sitting very ungracious and very silent with us...

Clearly, two of Austen’s pet peeves were badly behaved children and indolence. Even though the gentry could sit around a good deal of the time doing nothing, it was frowned upon for they were expected to be engaged in useful employment of some kind and Austen’s disapproval of those pet peeves are reflected in her characters:
Some of the parents/guardians/adults guilty of spoiling or lacking control of their children were Lady Middleton, Mrs. Bennet, Mrs. Norris, The John Dashwoods, and Mary Musgrove. Some indolent characters were Mr. Bennet, Lady Bertram, Sir Walter, and Mr. Bennet. I would say most, if not all, lack Austen’s and the reader’s admiration.

In the context of the story, however, the Dashwood girls do come off as a bit snobby but on the other hand, perhaps the time frame of their lives should give them some leeway. (I like to think that Elinor, at least, is grateful even though she censures the Middleton’s society.) And since their feelings seem to align with Austen’s, should that count in their favor or should we perhaps look on Austen as a bit snobby herself? Hmmmm, I’m thinking on it.