How to set custom user view mode for comments
The variables {{ author_picture }} and {{ author_name }} are prepared in preprocess in the hook template_preprocess_node(). You can modify these variables or add your own in a custom preprocess hook.

Or you can make the field uid configurable in UI by modifying the field definition in the node entity:

mymodule.module

function mymodule_entity_base_field_info_alter(&amp;$fields, $entity_type) {
  if ($entity_type-&gt;id() == 'node') {
    if (isset($fields['uid'])) {
      $fields['uid']-&gt;setDisplayConfigurable('view', TRUE);
    }
  }
}


and you'll find the Authored by field in the Manage Display page, where you can configure how the referenced user entity is formatted (which view mode is used).

A third and IMHO perferable option is to use the module Display Suite.