Mobile devices can have all kinds of screen resolutions, from small low-end phones to large tablets. While the available resolutions on iOS are manageable, the markets for Android and Windows Phone devices is so fragmented that you can get pretty much anything.
That means you should try to develop as resolution-independent as possible. When you can, you should develop your game in a way that it checks the screen resolution and automatically resize the graphics accordingly, so that the part of the game world shown on each device is the same.