How your Masthead background appears on mobile or tablet devices depends on multiple factors such as:
Content: generally, we don't recommend actual images with text as the image needs to be resized/cropped for the mobile screen.
Masthead Background Settings: depending on the option you selected here, this will either crop your background image or have it display fully on mobile. This can leave a space above and below the image on mobile devices. We recommend changing this setting and checking the mobile version in Preview mode as you customize your site.