Miranda does have a face for high fashion but probably only for very specific labels that have an aesthetic that suit her look.
She could easily rock shows like Chanel, Valentino, Gucci, Versace, OdLR, Michael Kors and Chloe.
But if you put her somewhere like Givenchy, Balenciaga, Rodarte, Lanvin or Prada she'd look completely out of place. So even though she has more than enough potential to do high fashion, what she'd get out of it in the end is worth too much hassle when she can just do a VS catalogue and earn just as much.