A secondary camera, mounted on mobile devices facing user, is primarily for video calling or nowadays being used to taking selfies. Usually the distance of subject, user, is not far from the phone hence to keep the cost low most companies mount a low resolution (less megapixels) secondary camera. Cheaper the phone is lower the resolution is.
Other things that contribute in poor picture quality is quality of lens, type of camera e.g. VGA camera, poor lighting etc.
