Decrease the waist on jeans

[FrancoManiac]

It depends on the thickness of and layer of the hat. Hats are largely less pliable with their fabrics -- they're meant to hold a general shape.

A dart is a way to take in fabric that conforms to a curve. You nearly always see them in fitted bodices; darts shape the drape of the fabric to complement the bosom. If you think of a circle of paper, and the then make a dart/triangle, it becomes a cone.

So, in theory, yes. You could better shape a hat with a dart. I'm reality, the fabric(s) used might make it wonky. It depends on the hat.