It's important to note what Palpatine was doing at the time. He was trying to convert Luke and get him to replace Vader as his apprentice. Luke considers his lightsaber a symbol of his new Jedi Knight status. Vader confirms this as well. The Emperor is breaking that symbolism by noting that his father made a lightsaber too yet he also serves him now. He is painting the picture that Luke's fall is inevitable. He is not concerned with Luke's aesthetic design choices.