An empty string is used for the glue parameter of implode(). get_post_class() will return an array, and you want to separate the array values with a space, so use this:
$post_class = implode( ' ', get_post_class() );
This appears to be a bug in the WP Most Popular plugin. I submitted an issue to the wordpress.org forums here. Hopefully the author will fix the bug, otherwise the fix posted above will be overwritten if the plugin is updated.