I personally don't think a mobile interface or app will allow you to produce a high quality cover. I can barely text coherently, let alone do image editing. Mobile apps are very specific to their intended outcomes and don't always allow for freedom of artistic expression when compared to computer programs, in my experience. I'd recommend sticking to the desktop/laptop approach and utilize programs such as
gimp which is a fantastic free image editor with bells and whistles to rival Photoshop.