Feature for feature, Nena is probably better-looking than her daughter:
Uma's nose may be too big for classical beauty, her lips may be too full, and her eyes almost too hooded.
But somehow, with Uma, those flaws added up to an even greater appeal. The almost exaggerated features also seem to signal a wanton lasciviousness that gave her more personality.
Uma seems to have more star quality than her mother. That's probably an unfair comparison, since Nena worked in the 60's, and at the time a model's job was to look somewhat blank.
But maybe part of the reason Uma got the opportunity to be a movie star was because of her expressiveness. At her peak, between the ages of 24 and 31 (roughly from Pulp Fiction to Kill Bill), she was just spectacularly beautiful.
Here she is in one of the iconic shots from Pulp Fiction, expressing wryness -- which implies intelligence and a sense of humor:
When the role called for it, she also had a certain determined cast to her mouth and jaw, that gave her a certain defiance. Kill Bill was just a comic book brought to the screen, but it needed a tough, sexy, tragic heroine, and Uma was perfect.
Take a look at this interview with Charlie Rose, which took place close to the release of Kill Bill. There's no point in the entire episode at which she's not just luminously beautiful, and she doesn't even appear to be wearing much makeup.
Anyway, it was interesting to find out that her mother was a great beauty, too, though I suppose it shouldn't have been that big a surprise.