I don't think Tatiana is a great beauty on the level of the Grimaldi/Casiraghi women-to be fair not many women in Royal Europe are imo. I am also not a fan of her fashion style.
But I do think she is lovely and has some of the most beautiful hair I have ever seen. And unlike many of the people in her circle she seems approachable and genuinely warm. I remember when she accompanied Andrea to the Philippines some years ago on behalf of AMADE. There were photos of her touring a particularly awful slum area and the look on her face as she held a poor woman's infant was a look of genuine distress and empathy. You cannot fake the expression I saw in that picture.
Those are the qualities I look for in a Royal and an average person..a sense that they are genuinely in tune with others, compassion. It can make the homeliest woman appear radiant imo.