Cycle comments on field--comment.html.twig
You can try something like this:

   {% set pre_render = comments['#pre_render'] %}

   {% for key, item in comments if key|first != '#' %}
     {% if key != 'pager' %}
       {% set item = { '#pre_render': pre_render, 0: item } %}
       &lt;div class="comment-{{ cycle(['odd', 'even'], loop.index0) }}"&gt;{{ item }}&lt;/div&gt;
     {% else %}
       &lt;div class="comment-pager"&gt;{{ item }}&lt;/div&gt;
     {% endif %}
   {% endfor %}