MediaWiki search uses the IN BOOLEAN MODE modifier in the MATCH AGAINST clause. MySQL does not rank the results when the BOOLEAN modifier is used. I've improved the ranking significantly by adding the clause ORDER BY MATCH($field) AGAINST($searchon) DESC. This ranks the results without the BOOLEAN modifier.
In the function queryMain in the file includes/search/SearchMySQL.php, add the following entry to the $query array:
$query['options']['ORDER BY'] = substr($match, 0, -18) . ') DESC ' ;