Buffy Sainte-Marie is beyond a *quality artist and human.

Boosting this quality contribution up in the Hot Listing - declared quality by Ashenkase.

Why is the instrument used in the mouth?

It looks to me like you vary the pitch by changing the tension on the string.

Cool song though

Seemed to me the pitch doesn't change until 1:32, but the sound does based on her mouth shape, like a jaw harp.

In the United States a musical bow is primarily found in the Appalachian Mountains, where it is called a "mouthbow" or "mouth bow".

https://en.wikipedia.org/wiki/Musical_bow

Putting it up to her mouth acts as a resonator adding volume but it also adds the overtones that the mouth cavity can create a la Tuvan Throat singing or Tibetan Monk singing to add notes from the overtone scale.

sesame street.... kiss her on the mouth , sweet as any wine, wrap herself around me like a sweet potato vine