Image lightened enough to resolve the shape of her coiffure.
Portraitist Justus Sustermans was Flemish and court painter for the Medicis. His Wikipedia article is here.
Margherita de Medici married when she came of age in 1628. This portrait shows the increasing sophistication of lace collars at about this time. She appears to be wearing hanging sleeves and a large metal brooch on the sleeve to the right. I cannot identify what is to the right of her head.
Keywords: Sustermans, Margherita de Medici, Medici family, Farnese family, Princess, Duchess, Italian, curly coiffure, hair ornament, high neckline, collar, lace, epaulettes, hanging sleeves, necklace, draped necklace, earrings, brooch