It’s unimportant… unless the shipped item later turns up missing, and the Postal Inspectors have to try to find it, or find out where it went off the rails. Since they can’t find “Tom Swift” – and they will get the reference as soon as they see that signature on their receipt – then it will look to them like fraud, and you could be in trouble if the postal worker is able to point to you and say, “That’s the guy who signed,” and obviously, you aren’t commonly known as “Tom Swift”.
Then you’d have to do some fancy footwork to either produce the item or explain that you actually delivered it to the intended recipient – and got a receipt with a valid signature on it. In other words, confronted with a missing delivery item and an obviously false signature, what would be your initial supposition? They’d have the same suspicion at that point.
If you had signed as ”@Nullo”, then when the Inspector and postal worker confront you, at least you’re working from your real name, and they won’t suppose that you were attempting to steal the delivery, since you obviously used your real name.