If this is the official namespace there is no option to shorten it except from aliasing the namespace with the `use` statements (which even would require more characters):
use PHPMailer\PHPMailer\PHPMailer as Mailer;
$mail = new Mailer;
F3's autoloader (AUTOLOAD) supports only "full qualified namespace to directory" mappings. The autoloader of Composer supports "shortened namespace to directory" mappings (
Example). It could be possible that there are more autoloader libraries supporting this feature.