How to add a class to more link of a view?
You can do it by implementing template_preprocess_views_view() in your theme under THEMENAME.theme file as shown below:

/**
 * Implements template_preprocess_views_view()
 * @param array $vars
 */
function THEMENAME_preprocess_views_view(&amp;$vars){
  if(YOUR CONDITION FOR ADDING A CLASS){
    $vars['more']['#options']['attributes']['class'] = array(
      ENTER YOUR CLASSES AS ARRAY ELEMENTS
    );
  }
}


Reference: http://www.drupalthemez.com/blog/d8-adding-class-more-link-view