The Librarian in Black is ready to take names and kick butt over at the NY Times style section for suggesting that the antithesis of a siren is a librarian. And God bless her for it. But she briefly touches upon poor Miss Marion Paroo.
I think it's rather funny, the "Marion" stereotype ... it's one of my favourite musicals. Thing is, Marion is dressed no differently from any other female character in the movie, given her age and marital status (she's not married and well-off, so she doesn't get a richly feathered hat). In fact:
1) Shirley Jones was pregnant at the time (which is why she doesn't do any of the really strenuous dancing in the movie -- no leaping off of tables or hanging off of balconies) and towards the end of filming came close to literally having to be sewn into her costumes. And she's not exactly wearing a potato sack or an empire-waist dress that hides her curves.
2) The reason why the whole town is against Marion and why Harold Hill throws all of his charm at her is because they all believe that she is a fallen woman [the paramour of the late Mr. Madison, town benefactor] who is trying to corrupt their innocent children with works from Rabeleis and Balzac. We don't know if Marion has a library degree, but she is obviously educated, and there seems to be the implicit notion by the townspeople that an educated woman is a tempted woman (we don't see anyone who identifies as a school teacher in the town, so we see no other examples of educated women).
Yes, she looks very prim, but in pre-WWI Iowa, I daresay most every woman (except for a couple of particular subsets) would look just as prim. I think Marion is the bee's knees. And you know she kept "Prof." Hill on his toes ... the woman has access to the works of Sir Richard Burton, after all.