Based on inteviews with Valerie Tripp it was always my understanding that they made the physical characterstics of the dolls, and many of the outfits, based on what the author wrote in the story.

I imagine that the author was given something like - we want a story about a homeschooled girl who goes on an adventure- then the author came up with the character and then the doll was created.

That's just what I would think.