Your image resolution may simply be too low. The recommended image resolution for printing is 300 dpi. 

In Sketchbook Pro on desktop you can adjust your dpi in the Image > Image Size menu. 

In Sketchbook on mobile devices, dpi can be set in the Edit Canvas menu, which is unlocked with the Premium Bundle‍.  

If you do not have the Premium Bundle unlocked, your file will export with a default 72dpi. There are free online image editors that can set dpi for an image but you will want to set up the canvas for the correct pixel dimensions when you start. For more information, see setting the maximum canvas size for printing‍.