No, it's not a criminal offense. That's why overstayers go to a detention center and not a prison. They are not charged with overstaying and put before a court for sentencing, They are detained until being deported. Like I said, they haven't got the resources to do a clean sweep. When you read a news article about a guy who has been caught for something like 8 years overstay, one wonders why /how he could just cruise around Thailand for 8 years. You would think that after, say, 1 year overstay, an immigration officer is assigned his case and they go after him, but once again, lack of resources.