add_megamenu_styles_to_admin.php

<?php
function JMA_child_admin_enqueue_css_js()
{
    $upload = wp_upload_dir();
    $upload_dir = $upload['baseurl'];
    wp_enqueue_style('childmegamenu', $upload_dir . '/maxmegamenu/style.css');
    $output = 'a.mega-menu-link {pointer-events:none!important}';
    wp_add_inline_style('childmegamenu', $output);
}
add_action('admin_enqueue_scripts', 'JMA_child_admin_enqueue_css_js');