No, I do not believe if you found a cure for cancer that it would cure diabetes as well. It is possible, but not likely. Cancer is caused by many things including genetics (for some people). Diabetes is caused in some cases, by genetics, and in others insufficient production of insulin by the body. While there is a similarity in that cancer and diabetes may both have genetic components, there are many different kinds of cancers and many different possible causes. Additionally, just because the cure could be a genetic one, does not mean that it would be the same genetic issue as diabetes.
If I somehow magically became endowed with the medical knowledge required to cure cancer, I would come up with a cure for cancer. It wouldn't cure diabetes, because that's an entirely different disease. I might then work for a cure for diabetes, however. I'd also work to cure AIDS and world hunger.
Finding a cure for cancer wouldn't cure diabetes. Both diseases need different treatments and effect different parts of the body. Cancer and diabetes are very different diseases, and curing one wouldn't cure the other, unless the treatment was a cure all that helped prevent and cure all diseases and illnesses, but that's highly unlikely.