Adding a new variable to account menu for twig rendering
In your THEMENAME.theme

function THEMENAME_preprocess_menu__account(&amp;$variables) {
  $variables['account_name'] = Drupal::currentUser()-&gt;getAccountName();
}


And in your menu--account.html.twig 

&lt;div class="welcome-message"&gt;
  Welcome, {{ account_name }}!
&lt;/div&gt;


Just tested and it works, so basically what you posted should work but probably you mistyped the twig file name which needs two dashes (--)