When shaping the neckline we usually start with binding off a number of its center stitches. After the neckline is finished there always is a visible gap between the first bound off stitch and the shoulder stitch next to it.
So I decided to take photos of the steps that will help you to make the neckline smooth and pretty. Sorry for the shiny acrylic thread in the yarn.
Finished back of the doll's cardigan (this is the actual back of the Dirndl cardigan).
The clearly visible gap between the bound off stitches and the shoulder.
Get the end of the yarn that was used to bind off stitches and thread it into a tapestry needle.
Insert the needle through the 1st BO stitch ( the needle should go under both loops of the stitch) going from back to front, pull yarn.
The needle with yarn are on the right side of your work.
Find the very first stitch of the shoulder and insert the needle in it going from front to back.
The needle should go under both loops of the stitch.
Pull yarn to the back closing the gap between stitches.
Yarn on the back, the stitch gap is closed. Secure the yarn end by sewing it in on the wrong side of the work.
That's how the wrong side will look just before you trim the yarn off.
And this is the front view of the finished result. The neckline looks nice and even.