I have been thinking about this, and I believe we tend to facilitate a false dichotomy between profession and action. What we confess and profess is essential to our Christian Faith, as much as what we do. Orthodoxy is a Faith of creeds, and the Divine Liturgy is the vocal profession of our Christian beliefs. And, of course, our actions should always accompany our professions.
But Christians have always used symbols and signs as a way to encourage other Christians, to witness to the world, and to remind themselves to live a Christian life. After all, Orthodoxy is an iconic Faith. So, why begrudge those who display bumper stickers such as the one mentioned? Truck drivers have a tough job, and they face many temptations. Truck stops often attract prostitution and drugs. Christian truck drivers out on the road alone are extrememly vulnerable to satan's schemes, so I think it is encouraging that many of them display outward messages that speak to their faith.
When we see such things, I would think that we should pray for the individual displaying the message, and pray for ourselves, that we would live lives glorifying to Christ. Sometimes I can be guilty of being a cultural snob, mocking that which I view to be "kitsch." But if it helps the person displaying the message or encourages others in some way, then who am I to demean it?
That's my two cents, FWIW.