It’s only a Nazi propaganda film because their team lost. Objectively her work and especially Triumph of the Will is a classic of film and staging technique that would have won multiple Oscars if they had such a category.
In the context of this post you should be very careful about conflating your interpretation of a person’s character and their work. In main because you will quite often get more variance in estimates of a persons character than in the technical or artistic merit of their work.
Leni Riefenstahl was an excellent technician and also a brilliant artist. There is no doubt of that.