First off, I like how your general skull shape coincides with the stick people on the cards. They need some work, but that's a nice symmetry to try and keep.
Zorg has a point about the shapes being potentially uninteresting, but I think it can work as a blocky style if you increase interest elsewhere. The triangle eyes look really good; they draw interest to the lower part of the face, which is counterweighted by the bulk of the forehead. The teeth on the outside of the skull help to anchor and balance it, too.
For the shadows, just keep in mind the shape of the skull and what perspective you want to imply.
On a side note, I think you should reduce the cut on the corners of the cards. That's a pretty significant area, far more than you'd see on a real-life card. Unless you're pulling some
Battlestar Galactica sh!t.
I'm gonna end with throwing two iconic skull designs in here. Also check out Halo ODST's Superintendent and the Titan games by Puppydog Studios, they are similar and may inspire you.