Well, according to all my maps - not to mention my memory of going there from Hammamet - the distance is actually about double that - roughly 230 miles each way, and certainly not feasible in a single day, especially given the fairly basic state of many of the roads. When we visited Matmata from Hammamet it was part of a 3-day "safari", and we didn't reach Matmata until the morning of the second day. If you can spare the time to do it it's a worthwhile trip, but if you're only in Tunisia for a week it's a huge chunk out of your holiday. We were there for 3 weeks on that occasion, so it wasn't so bad.