The open mouth refers to the site of the body where, as Barbara Kruger puts it, "the dry outside meets the wet inside".
It is the part of the body we use to feed ourselves, make love and express joy or rage. It is also where language exits. These are all things that relate to existence. But the mouth can also be violated and penetrated, sexually, medically, punitively..... so when one reads the visual language of an open mouth, all these layers and possibilities come into play.
In many ways, the open doll mouth can be as complex as the human one.