This particular section seems to be the "see how silly Marianne is" section. From her descriptions she's supposed to be somewhat kind and sensible, but from her actual portrayal she's less so. The delight she has in jumping on the "let's mock Brandon" bandwagon, her readiness to accept a *horse* as a gift that she should know they can't easily afford, and so on... of course, she's young, but she comes across as pretty silly as well, at least here.
It's hard to tell, without pretty much being a part of that society, just how much of a killjoy Elinor is supposed to be, or how "correct." I mean, she seems a bit stuffy by our standards today, but then that was probably proper and accepted back then.
Willoughby at least comes off slightly better than Marianne - still a bit silly and flighty like her, but at least he seems to have the self-realization of it, at least that's the feeling I get from his so-called three reasons to dislike Brandon - he sounds like he's making fun of himself, and fairly intentional in his lightheartedness.
And poor poor Colonel Brandon. Ah well.
Notes: I figure most of these aren't needed (probably none of them), but what the heck:
Marianne's brownness: this is of course meant to be a negative. Despite our love of tans these days and the "healthiness" of being an outdoorsy person, in this society paleness was still to be aspired to, and rambling about out of doors enough to get tanned was inelegant. (As was running, as the girls thought themselves excused in doing in the last section to try to get home before they got too wet.)
Names: The reference to "Miss Dashwood" would be to Elinor specifically. In families with more than one daughter, the eldest (unmarried) girl would be "Miss Lastname" and any younger sisters would be Miss Firstname Lastname." So Elinor would be Miss Dashwood, while her sisters would be Miss Marianne Dashwood and Miss Margarat Dashwood. And of course using a Christian name alone, especially without "Miss" in front of it, would show a high degree of familiarity in this formal society, and would usually indicate an engagement or blood relation or very close friendship with someone of the same sex, or something similar.
Natural daughter: As might be inferred from the context, a natural daughter would be basically an illegitimate/bastard daughter. Something not really acceptable in this society, but obviously more a peccadillo for a man, while it would be much more serious for a woman.
Schedule for further readings, in case anyone needs a reminder!