The film is about Natasha, Yelena is a textbook example of a supporting character. Everything Yelena does in the movie is connected to Nat. The main plot revolves around Natasha making up for past sins and coming to terms with her "family". Moreover Natasha gets all major action scenes, in fact Yelena doesn't have a single one of note besides fighting Nat, and well, Nat is also involved in that one. Literally everything points to Natasha as the protagonist.
Do we get introduced to Yelena as a character? Of course, that alone doesn't make it her movie though.