Black Widow was introduced in
Iron Man 2 and wasn't taken seriously by the other characters until she beat up Happy. Her main purpose in that movie was to beat people up. Black Widow does have the benefit of being a female hero with the most screen time so she was aloud to grow beyond this.
Monica, yeah you're right she's also not in that boat.
Wanda is more complicated but she also was introduced as a terrorist and unleashed Hulk unto a city. (I would count anything the characters did in
Civil War because the premise of that movie was that they fight each other.)
In WandaVision she psychologically tortured an entire town of people. Granted, she wasn't totally aware of it for a while, but she also tried to turn Monica into paste at one point when Monica just wanted to talk (like she didn't know she had powers). Meanwhile Vision never tried to hurt anyone. Wanda is an interesting character but she's also a deeply problematic one, but not more than Tony Stark I suppose.
(Agents of Shield isn't MCU
same with the Netflix stuff, but the characters on that show do have the benefit of being in a TV show with more screen time to flesh them out.)
Pepper Potts and was introduced as a supporting character and love interest, and not a hero. She wasn't as hero until Marvel realized they needed more female heroes for their
Endgame group shot and to have her be there to say goodbye to Tony as he died.
Carol's whole reason for being in
Endgame was for fighting. And a the end of her movie she was shown taking pleaser in killing the Kree (honestly that's whatever, I was as emotionally numb during that movie as Carol was). Her personality is also a little bit...cardboard-y...
A lot of female heroes are introduced through violence in the MCU.
Peggy decked a cadet for proof as to why she's his worthy to be there, while Steve is shown worthy by being willing to take a grenadine for his comrades. And I get it, 40's, sexism, but there are women who fought in WWII (and throughout history) who where also belittled despite their service and they certainly didn't prove their worth by decking their peers or subordinates, which makes her depiction stand out even more.
Peggy also held a gun to Cap and shot at him without warning for proof of the shield working...
So I suppose, it's more than personality I am referring to but overwhelming abundance of aggressive behavior from women. It might not always take the form of a quick-to-anger personality, but it is usually in the form of aggressive behavior. And that aggressive behavior is usually used as a justification tool to show that they are a "girl-boss".
And to see that from Wonder Woman in JL in the form of the terroirst scene (that we've all talked to death, lol) none of the other characters were introduced by killing people.
That really says something.
That would definitely help, yes. We still only have female led 3 blockbuster compared to the dozens of male led ones. Having women still being primarily kept in supporting rolls does nothing to help their character.
And again, I don't have a problem with any of these characters alone, and the more screen time they get the more layered they will become (I should hope). Xena herself probably would have been depicted similarly if she was limited to supporting roles and team films.
Edit: MCU women tend to fall into the category of "love interest" or "girl-boss". Same with the DCEU, they are certainly not without this problem too. Wonder Woman is the exeption for both franchises (in her films).