I assume you mean someone who dresses manikins for shop windows. Such a person is called a 'window dresser'.
Manikins
NOOOO! Manikins are obv plastic. So the makeup will be different on the manikins than on human skin. So tbh wots the point? It ain't gunma work.
Pediophobia
If you are speaking of live models, they are called manikins. Artist models are supposed to hold the pose that they are placed in for long periods of time. Since this has been a practice for nearly every artist that there ever was, the term carried over to plaster dummies. Dress dummies are also called manikins for that very reason.
You can purchase just about any time of manikin you can possible think of. From full body educational manikins, to half body manikins, to modeling manikins for places like stores.
Janet Cofer has written: 'Manikins for the small museum' -- subject(s): Museum manikins
That person is called an embalmer or a mortician. They are responsible for preparing the deceased for funerals by dressing them and applying makeup to make them look presentable for the viewing.
There are many ways a person can learn how to make bustier dresses. A person can learn to make bustier dresses at popular on the web sources such as Foundations Revealed and Stay Lace.
Manikins
Those dresses are called "fit and flare dresses" or "mermaid dresses." They are fitted at the waist and flare out at the bottom, creating a slimming silhouette.
A typical person who dresses casually
Mermaid dresses