There is no
wp_comment_count in a
WP_Query object, either in the "top" level as in your code--
$popular->wp_comment_count-- or in the individual post objects which is what you actually want to be checking.
There is a
comment_count in the post objects though, and that is what you want--
Your code should look like this (cleaned up and formatted into something readable):
while ($popular->have_posts()) :
<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a> </li><?php
echo $post->comment_count; // here is your count
Notice that you are checking
$post for the comment count.
$post is a global set for each item in the Loop by
$popular->the_post(). If you are trying to display the count, as opposed to do some other operations based on the comment count, you should probably use
comments_number as in the answer by @NathanPowell