I don't mind the idea of the Mary character as much as the way Moftiss shoved her down our throats for two seasons. She appeared in seven out of thirteen episodes, counting TAB. They gave the character far too much influence considering her almost non-appearance in the ACD stories and, whilst I know nothing of Amanda Abbingdon as an actress, she was not at all convincing as an assassin. Putting a gun in a character's hands does not an assassin make, and her unfamiliarity with firearms was glaringly obvious. She appeared to be a slightly podgy middle-aged housewife dressing up as a movie idea of an assassin, which is fine if you're attending a Halloween party, but not if you're attempting to convincingly portray a contract killer.
All four series of the show occasionally displayed a marked lack of convincing physicality, so the actors and actresses are not solely to blame, but in no way did she rise above the inadequacy.