For that you will have to open certificate.php which is inside
/certificate/type/YOUR CERTIFICATE TYPE/
Open the certificate.php file and look for lines that start with certificate_print_text and add the following line in the place where you want the username of the user (you might have to change the numbers 72 & 24 depending on the position and size of text):
certificate_print_text($pdf, $x, $y + 72, 'C', 'freeserif', '', 24, $USER->username);