Hi It does sound like Mirtaz has given some adverse effects to your mother. It would also be helpful to know the extent of benefits your mother has had with Mirtaz. It should be a balance of risk & benefits. Tapering Mirtaz as you are planning is ok, but I would recommend  review by a Psychiatrist before you do that, to prevent any deterioration in her mental health. May be she needs cross tapering of Mirtaz to another antidepressant. Hope this is helpful. Best wishes