One should wear a seat belt, of course. But that's not the question: the question is whether the State should punish those who do not wear seat belts. Given that they risk only their own lives (modulo the vanishingly-small number of folks injured or kill by bodies flung from crashes), I believe that the answer is unambigously no.
Wear a seatbelt. Don't fine and/or imprison those who do not.