This is incorrect. Your inner query is going to return the tag_id with the maximum value, not the one that is associated with the most statuses. The following would work:

SELECT tags.tag  FROM tags INNER JOIN statuses_tags ON tags.id = statuses_tags.tag_id INNER JOIN statuses ON statuses_tags.status_id = statuses.id GROUP BY statuses_tags.tag_id ORDER BY COUNT(statuses_tags.tag_id) DESC LIMIT 1;