Would my mom's death be medical malpractice?

Your mother may have died as a result of medical malpractice.   unfortunately, you have provided inadequate facts in order for me to reach a more definite conclusion.  The standard of practice requires for a patient receiving Coumadin to be tested frequently in order to assure that the level of anticoagulation is therapeutic and not in excess of that needed.  It is common practice if a patients INR on a particular dosage of Coumadin is stable to extend the time between testing.  The timing of your mother's testing and whether it was appropriate depends upon whether she was in fact stable or not.  You also have omitted information concerning the type of cancer your mother was suffering from, the stage of the disease and her prognosis.  These are factors that are important to the decision of whether it is practical to pursue a legal claim for medical malpractice.  It is important that you of obtain the services of an attorney experienced in handling medical malpractice claims of this type.