She isn’t royalty. She has no idea about the culture, nuances, or skills it takes to rule a nation. Unsurprisingly her marriage ended up costing Prince Harry his place in the royal court.
She is hardly the first non-royal to marry into Britain’s royal family. Her Late Majesty Queen Elizabeth The Queen Mother was not royal by birth, but she was one of the most popular members of the royal family. Current members of the royal family who were not royal by birth include HRH The Duchess of Cornwall, HRH The Duchess of Cambridge, HRH The Countess of Wessex, Vice Admiral Sir Timothy Laurence, HRH The Duchess of Gloucester, HRH The Duchess of Kent, and HRH Princess Michael of Kent.
She is no longer a Duchess so that title doesn’t apply to her.
Meghan is, and will remain, a Duchess of the United Kingdom. Her formal title is Her Royal Highness Princess Henry Charles Albert David, Duchess of Sussex, Countess of Dumbarton, Baroness Kilkeel. When they step down from royal engagements at the end of March, Harry and Meghan have agreed that they will voluntarily relinquish use of their royal styles and titles. It is not yet clear by what titles they will be known, but all that has been agreed so far is that they will not use any
royal styles or titles, i.e. Royal Highness and the titles prince and princess (contrary to common belief, Meghan is a princess by marriage, and hence is entitled to the title Princess Henry, not Princess Meghan). A dukedom is a peerage, not a royal title. The only means by which a peerage can be revoked is by an Act of Parliament. There is absolutely no suggestion that the British Parliament intends to pass legislation to revoke Prince Harry’s peerage.
It is not particularly uncommon for members of the royal family to use lesser titles than those to which they are technically entitled. HRH The Duchess of Cornwall is technically the Princess of Wales, but she chooses to be known by the lesser title of Duchess of Cornwall. The children of the Earl and Countess of Wessex are technically HRH Princess Louise of Wessex and HRH Prince James of Wessex, but they use the lesser titles Lady Louise Windsor and Viscount Severn. Harry and Meghan’s son is known as Master Archie Mountbatten-Windsor, but he is technically entitled to be known as Earl of Dumbarton. The Duchess of Kent, although formally styled as a Royal Highness, is known as Katherine, Duchess of Kent or simply Katherine Kent.