My grandma had a lot of dolls in many shapes and sizes and my experience in collecting dolls has been that most of them have closed mouth expressions. It wasn't until AG that I was aware of an open mouth smile. (Except for Barbie).
Consider the antique dolls from Germany and Paris circa 1880, or Ideal and Revlon 20" Fashion dolls, Madame Alexander, even Kewpie dolls all have closed mouths. In fact the majority of doll brands (economy or premium) HAVE closed mouth expressions today. A small list includes:
Here is a gallery of dolls with closed mouth expressions.
Closed mouth dolls are popular in the European play doll market. (Kidz n Cats, Sasha, A Girl for All Time, Bonnie and Pearl, etc).
Kids n Cats and A Girl for All Time are fully articulated which offer much pose-ability when taking photos, whereas the USA market consistently produces only five pints of articulation, (per the above list).
Vinyl and cloth play dolls mean just that. The dolls are meant to be played with, positioned, posed and won't "break". Collectible dolls (porcelain, bisque, antique, paper mache, etc) are just that - on the shelf pretty to look at.
Check your collection for the comparison then post a comment with the percentage in favor of closed or open mouth smile. I'm curious to how the numbers turn out.