For now it's just a fourth wall break because none of the players questioned it.

If they do ask, the lore reason is:

"DM" stands for Dunnusaidu Marudu, an ancient philosopher and the inventor of hedonism in this universe. When he died, he decided that even though he has experienced all the possible sensations that his current body and the current time period can provide, he still hadn't had enough. He had enough followers to become a minor god, and his spirit wanders the world, possessing random people for a short while to gain new experiences. He is not malicious which is why he created this talisman that appears on those possessed by him, he usually doesn't speak much when he does this, and he never makes major life decisions in the given body.