What is Duchess Meghan’s constitutionally appointed role? And how has she been performing it so far?

She is a Princess of the United Kingdom. It is difficult to perform the role badly, but Princess Sarah Fergusson and her daughters somehow manage it.

There is no constitutional role. Members of the official working royal family support the monarch by taking on official ambassadorial duties, charitable, educational, military, medical foundation works.

https://www.royal.uk/role-royal-family

Meghan, Duchess of Sussex, appears to be doing well. She put out a cookbook in order to help one of her charities earn money and it became a best seller.

She has none since her role is not "appointed", she was granted the title as part of her marriage however neither her, nor her husband, have any constitutional, i.e. governing roles in any country. As a member of the Royal Family they are expected to attend to certain ceremonial duties but in no way have anything to do with governing.

She does not have one. She married in to the Royal family, and could be expected to do Royal sorts of things, which are constitutional in the unwritten part of the constitution, i.e., Royals leading by example, but, that's about it. She has zero influence on what the government does, and now power whatsoever. At this point, neither does her husband.

No constitutional role.

She doesn't have one, so there's no way to comment on how she has performed it.