Jesus could have had a ball with it, given his penchant for directing special criticism toward religious people and their overt expressions of piety. Smudged foreheads are public expressions; as Jesus knew, all public religious statements can bear witness to the gospel or ensnare us in games of spiritual self-congratulation.
Preachers and the liturgies they employ need to find ways of explaining the purpose of Ash Wednesday and Lent, for these observances are postbiblical inventions. A sermon on this Matthean passage might take up the topic of how people conduct a devotional life as a means of communing with God. In the text Jesus highlights the pitfalls that accompany religious behavior, warning that piety intended to garner notice from others is really no piety at all.