How to enable visual editor when editing comments on the dashboard?

If you are using WordPress 4.0+ you can do this using the wp_editor_settings and the global $pagenow to determine if you are on the comments page.

add_filter( 'wp_editor_settings', 'remove_editor_quicktags', 10, 2 );
function remove_editor_quicktags( $settings, $id ){
    global $pagenow; 
    if ( $id == 'content' && $pagenow === 'comment.php' ){
        $settings['quicktags'] = false;
        $settings['tinymce'] = true;
    }
    return $settings;
}