Cassini flew by Mimas on August 2, 2005, and showed what had been expected: A battered world, with few if any signs of internal activity.
This leaves us with a mystery. Mimas is about 400 km in diameter. But moons of its size vary greatly. In the image below, from left to right, there is Neptune's moon Proteus, which could not even pull itself into a somewhat round shape, battered Mimas, the Uranian moon Miranda, which shows signs of intense internal activity that has now ceased (note the large grooved features), and Enceladus, which is still active today. It is indeed a complex solar system!