Vertex shading is forced by default on mobile to improve performance. This means you can’t have visible shadow maps since all materials will use vertex shading.
Also, consider rendering the viewport at a lower resolution using the viewport stretch mode and setting a relatively low base resolution. This improves performance tremendeously and is often needed to achieve good 3D performance on mobile. See Multiple resolutions in the documentation for more information.
The optimization tutorials are worth reading if you want to know more optimization tips.