Let's say on an iPhone portrait mode screen with 640x1136 resolution, if we tried to force a 1600x1200 cover photo into that panel, the photo will be extremely distorted. So instead, we have to maintain aspect ratio and center the cover photo around the focal point while filling up all the space on the screen. So there isn't really a standard ratio as it really depends on the screen size/device etc. The focal point is a great tool to use to ensure your cover photo will display perfectly on all devices including mobile.
What is a cover photo and how do I change it?
Cover Photo Focal Point.