A mortal wound is a very severe and serious injury (almost always a form of penetration or laceration) whether accidental or inflicted intentionally (by either suicide or homicide), which leads directly to the death of the victim. Death need not be instantaneous, but follows soon after.
Conversely a scratch could turn deadly if it becomes infected, especially under adverse circumstances. These include poor wound care, an elderly, diabetic or immune-compromised patient or a virulent or antibiotic resistant pathogen. However, unless the scratch was self-evidently and indisputably the cause of death, even after the fact it would be unusual to refer to it as a 'mortal wound'.
Severe contusion leading to death is more commonly described as a fatal blow or a mortal blow. Mortal wound is only strictly inappropriate when discussing a literal death not related to any penetrating or lacerating injury (by poison, for instance).