No, I don't believe it's a sin at all. I think it's sad because people have died to give them that freedom to vote, but within that they also have the freedom to not vote.
I guess that you could apply this (to yourself only though).
The New Testament definition of "sin" comes from the Apostle James: "Therefore, to him who knows to do good and does not do it, to him it is sin" (James 4:17 NKJV).