[Paal Gulli]

This was Michelangelo’s first statue, and he wasn’t yet 30 years old when he finished it. Genius will out.

It’s also the only work of art he created that he signed. The sash across Mary’s bosom has his name on it, and her left palm bears the letter “M.” At the time this was generally regarded as a vanity and he was mocked for it. After that he limited his vanities to occasionally putting his face in some of his paintings. It’s on the walls of the Sistine twice (or is it three times?), but Pieta was the only work he signed.

[MurphsLaw]

Not for nothing,, It doesn't follow that Michelangelo would have been mocked for vanity... if of all his lifes work he only signed ONE piece of art. A vain person would make sure to sign everything...
And if I hold my left hand just like Pieta Mary's hand, the crease folds of my palm also form a visible letter "M" as well- so I don't think that's verifiable evidence of a personal sort of inscription...
Now he did paint himself into paintings, that is true- but he also painted others, contemporaries of his back in the day, into his paintings as well- some flattering, others not so much...
Genius, no doubt though.