I have created a complete deck of cards in Maya all of which are a 2d plane. The texture shows up fine. Be sure you have your directory set up properly. I place all my textures in the same folder as the object itself. Inside the object folder I have a materials folder all my maya files for each card and all the textures used for those particular objects. Be sure your material is set to be a transparent/diffuse material. Also be sure your texture is a format that Unity can read like a .tga or a .png. Example of my directory structure below.