Is it necessary to get a leg amputation at the mid-calf because of a very bad ankle fracture that wouldn't heal?

Ankle fracture: You have not provided enough information to justify such a drastic procedure. There are ankle implants or ankle fusions that generally can resolve chronic joint pain post trauma.

Not really: Amputations are usully used when there is severe disease to the leg such as gangrene. It can be used when there is severe, chronic pain or the leg is just wasted.

If severe, yes.: Infrequently, a complex ankle or lower leg fracture my result in chronic wounds, malunion, fixed fusion or nonhealing. This is particularly troublesome for open (skin broken) fractures or bone infection. Below the knee amputation can end the cycle of infection, pain and most importantly the lack of ability to walk. Prosthetic legs can return function and significantly improve qality of life.

It depends: It would have to be a quite severe fracture to need an amputation. These days an external fixator can be put on to allow healing of most comminuted really awful fractures then revisional surgery is done later. Only fractures that cut off the blood supply to the foot or have a nasty infection would need an amputation.

A possible option: For sever irreparable traumatic injuries involving the ankle a mid leg or a below the knee amputation may be a viable option.