Black Widow, aka Natasha Romanoff, first appeared in the 2010 film.
"You look back at Iron Man 2 and while it was really fun and had a lot of great moments in it, the character is so sexualised, you know?" the actress told Collider.
"Really talked about like she's a piece of something, like a possession or a thing or whatever."
Black Widow has since become a significant figure in the Marvel Cinematic Universe, appearing in seven films, and will star in her own standalone movie next month.
Speaking about the character's evolution, the actor said she was treated like "a piece of ass" at first.
"And Tony [Stark, played by Robert Downey Jr] even refers to her as something like that at one point. What does he say? 'I want some'.
"Yeah, and at one point calls her a piece of meat and maybe at that time that actually felt like a compliment. You know what I mean?"
She added that her self-worth was "probably measured against that type of comment" - about her physical appearance - back then, but that she had grown to understand her own self-worth better since becoming a mother.
"I'm more accepting of myself, I think. All of that is related to that move away from the kind of hyper-sexualisation of this character," she said.
The 36-year-old also noted how things were "changing now" for young female actors in the wake of the #MeToo movement.
"Now people, young girls, are getting a much more positive message, but it's been incredible to be a part of that shift and be able to come out the other side and be a part of that old story, but also progress," she added.
"Evolve. I think it's pretty cool."
The new Black Widow movie was originally due to be released in May 2020, but that got put back due to Covid. It will now come out on 7 July in the UK and Ireland, and two days later in the US.
The new film isn't an origin story, but it does come before the events of the last two Avengers movies, Infinity War and Endgame.