The dictionary definition of mature is: "pertaining to or characteristic of full development" this seems to be contrary to whatever your definition of "maturity" is.
I wouldn't exactly call being overprotectiveness "mature." Most people see a overprotective parents as parents who fail at raising a child, but on the other hand a parent who exposes their child to the world and its dangers are more likely to raise a more respectable and mature child.
I wouldn't exactly call Fluttershy's diffidence, erm... normal. I'm pretty sure that there is a reason 'shy' is in her her name and that being shy is one of her main traits. Common traits are are considered mature are: handling pressure, handling fears, and acceptance of negative feedback. All of the aforementioned points are those that Fluttershy is lacking in.
Excuse me? I don't recall eating a lot, especially on one occasion to be an immature trait, or at least one that will make one less mature.
I agree.
I don't know about that. Sure it was the episodes moral to trust your friends and such, but I don't think that questioning your friends is immature.
Hmm... it seems that for these last three ponies you are only looking at the negatives. O won't list them due to the large amounts of instances in the show and that some of the events moral value is ambiguous. (e.g. All of Feeling Pinkie Keen)
All in all, it is important to remember that maturity is is an ongoing process and not a state. The characters in this show are growing and are getting more and more mature, it can be concluded that we cannot conclude which of the ponies is the most "mature" unless the show ends or something of the likes.
EDIT: Formatting