You can do whatever you want on your templates. If by widget you mean reusable code, you could use a snippet.

So this logic can go on a template or snippet:

  <?php $latest = page('blog')->children()->visible()->limit(6); ?>

  <div class="widget">
  <?php foreach($latest as $post) : ?>
    <a href="<?= $post->url()?>"><h2><?= $post->title()->html() ?></h2></a>
  <?php endforeach ?>
  </div>

Instead of fetching the post on the template, you could use a controller and just pass the $latest variable to the template.

Kirby doesn't output any markup, you have total control to over it :D The API for querying content is very powerful.