As for the Mystique and Black Widow comparisons. That is interesting. In general, I think it is obvious Black Widow is way more popular, not least of all because she is in more popular movies (not saying you have to prefer any of the Avengers movies to FC or DOFP, but... they make more money, they sell toys, they're a bigger deal culturally).
As for in terms of character, I actually will agree that Jennifer Lawrence's Mystique has a pretty great arc over three movies. If fans are able to ignore it is not like the comic book version of Mystique (which to be fair Days of Future Past explains in a very fun, clever way) then it works very well... at least in DoFP.
The irony is that while Lawrence has a good arc, and she never really overshadows anyone (the only movie where I felt she got a lion's share of screen time over most cast mates was First Class, and she wasn't a movie star back then), she completely was disengaged by Apocalypse. With that said, Scarlett Johansson also looked pretty sleepy in Captain America: Civil War. Civil War is great, but Johansson is given such little to work with that she is checking out at this point, kind of like how Samuel L. Jackson has been on auto pilot in Marvel movies after phase 1 ended. The thing is that while Lawrence didn't have a bigger role than McAvoy, Fassbender, or arguably Sheridan and Turner in Apocalypse... she still had a larger role than Johansson has ever had in these movies in Apocalypse. This is the one where she "went home." And she could not have cared less when she finally got there.
This is another problem with Apocalypse as a film. With that said, I actually do think the Lawrence Mystique is a better character, but she was kind of detrimental to X-Men: Apocalypse in the end. Black Widow, even if it is because she is usually wasted, has never had that issue.