Remove the core mobile.css file and add your own to your child theme

gistfile1.txt

add_action( 'wp_enqueue_scripts', 'generate_replace_mobile', 100 );
function generate_replace_mobile() {
	wp_dequeue_style( 'generate-mobile-style' );
	wp_enqueue_style( 'custom-mobile-style', get_stylesheet_directory_uri() . '/css/mobile.css', false, GENERATE_VERSION, 'all' );
}