Well I'm very glad to know that I'm not alone in disliking Marianne.

Up to a point I can forgive her for being like she is because she is a teenager - her sort of irrational thinking tends to go with the turf, and she has her mother egging her on.

But there are two aspects of her behaviour which I really dislike:-

1. The arrogance of a position which says "My way of seeing the world and behaving is the only acceptable way, and if you don't think like I do and behave as I do then you are wrong".
(Her accusation that Elinor is "cold-hearted" is a good example.)

2. Her hyper-emotional histrionics. I'm not advocating repression here. When it comes to genuine emotions I think it's healthier if people can allow themselves to really feel them, and sometimes it's good to share. What I object to is people ramping up their emotions beyond what is genuine (eg "feeding their grief"), and then splattering their emotions all over other people, and thinking that having such high-wrought feelings makes them finer human beings than more restrained individuals. (I could easily work up a rant about this! ;-D )

So was I a 'Marianne' in my youth? It was a long time ago, so I had to think quite hard about this, but I plead 'Not Guilty'.
I had my share of youthful joys and sorrows. On a few occasions I tried the 'feeding my grief' approach, but quickly found that it didn't answer, and I've always tended to keep my feelings to myself. I was never a drama queen, and knew from the year dot that other people might not see things as I do.
I identify strongly with Elinor - she's my favourite JA heroine - and have to work to find sympathy for Marianne.