To answer the last question first, no, the imposition of ashes is not Roman Catholic. The imposition of ashes is at least1,000 years old in Christian practice and has its roots in Jewish penitential and mourning rites (sackcloth and ashes).
To answer the first question, we mark ourselves with ashes in humility and repentance to acknowledge our sinfulness even in faithful confidence of God's forgiveness in Jesus Christ. We do not, however, mark ourselves with ashes to demonstrate our piety. Many parishioners will wash them off shortly after the Divine Service.
Here is a helpful article on the history of the imposition of ashes published by the Commission on Worship of the Lutheran Church Missouri Synod http://www.lcms.org/graphics/assets/media/Worship/COWInsert0205.pdf (see page 2).