-- ?php if ($noad) { print file_get_contents("headerleaderboard.html");} --?>
If you are happy as a pig in mud you are happy and having fun, and you might be surrounded by mess. Pigs really enjoy playing in mud, so this idiom is comparing you to a happy, muddy pig, but in a good way.
We gave the kids some finger paints, and they were happy as pigs in mud.