Showy commemoration? "This stuff" is deeply penitent and a somber remembrance of our mortality. People can wash the ashes off whenever they please and can remove it after the service if they want to. The mark itself is meant as a visible mark of our sin. Imagine walking around with tattoos of all the sins you've ever done written on your face for all to see. Doesn't seem like something to brag about to me and is meant to instill humility.