To use the dateFormat method you use the core helper.
<?php echo Mage::helper('core')->formatDate($customer->getCreatedAt(), 'full', false); ?>
The second argument accepts 4 options; full, long, medium and short.
The third argument is a boolean value and determines if you want to include the time portion of the date.
Output of each of these formats is shown below for enGB locale
Full: Wednesday, 23rd March 2016
Long: 23 March 2016
Medium: 23 Mar 2016