While your query works for this example, there being only one tag in the tags table, it would not work in all instances. First, to find the most popular tag, the tag with the highest count, you would have to order the results in descending order. Next, to show only the most popular tag you would have to limit the results to one record.

SELECT tag FROM statuses s INNER JOIN (SELECT st.tag_id, t.tag, st.status_id FROM tags t INNER JOIN statuses_tags st ON t.id=st.tag_id)AS a ON a.status_id = s.id GROUP BY a.tag ORDER BY COUNT(tag) DESC LIMIT 1;