You know what Iron Man can be front and center all the time but at least Iron Man was not generally known as Avenger villain and is actually an essential member of the Avengers from comics, cartoons, videogames? So I don't get why you are comparing that to Mystique.
This is not specifically targeted at you, cos your issue may just be Mystique being leader rather than on the side of good, but just using your quote for reference.
Having read up on a lot of analysis on the history of X-Men comics from a number of experts, I am more and more convinced that comic book fans (or at least fans of comic book properties) are quite a conservative bunch. This whole concept of Iron Man/Wolverine being shoehorned is ok because at least they were Avengers/X-Men, but villains like Mystique have to always stay villains. ALWAYS. It's just stagnant. It's preserving a status quo. It's the direct contradiction to what Chris Claremont was doing with his books, where he was always evolving the team line up and redeeming evil characters. Heck, he caught a lot of flak for redeeming Magneto and making him headmaster of Xavier's school, but it was arguably one of the best stories he has ever done.
It's this stagnation that has led to the downfall of quality in X-Men stories in general. Jean always comes back, Xavier jumps in and out of a wheelchair, mutants are always persecuted hence No More Mutants, Mystique must always be a villain despite how humanised she actually became under Claremont's pen.
Now people are perfectly entitled to have problems with JLaw's performance, and her being shoehorned as leader of the X-Men for profit. These are legitimate complaints.
But Mystique starting out as a villain and becoming good is just a natural storytelling process called character development. Mystique being leader of the X-Men? Eh. But still understandable because this trilogy was always a prequel trilogy surrounding Xavier, Magneto and Mystique, and this movie was always about the young X-Men, particularly Scott BECOMING the leader he was meant to be. People keep forgetting this. If anything, I admire Singer for sticking to his guns for the integrity of the story he is trying to tell rather than giving us the X-Men proper right from the start due to fan expectation (most of whom have no clue about basic storytelling as long as they just get what they want). Now if Mystique continues to be leader past this film, once Scott's arc is done, that will be bad. But JLaw is going out. She won't take over the franchise like Jackman did.
But then again, maybe it's not fair to single out comic book fans. People are just afraid of change, especially when it comes to childhood loves.
It is also interesting that the Internet has this weird short term memory problem when it comes to, well, literally everything. Singer generally (except poor Cyclops in X2, but obviously that will change), has shown skill with balancing ensemble casts, even when Hugh Jackman was arguably the lead in X1 and X2. He has shown it again with DOFP. Why does everybody keep expecting that Mystique will hog all screentime in this movie, especially when JLaw herself says that she has less work to do in this movie?
And its not just promotional marketing.... looking at the stuff that we saw, it really looks Jennifer Lawrence will be in human form for most of the time and it just baffles me a lot. Then leading the X-Men, telling them what to do.... as a X-Men fan, its not just very ideal. And this is Jennifer Lawrence's 3rd freaking X-Men film.... are they still letting everyone know that Jennifer Lawrence plays Mystique that she didn't need to wear the Mystique look for posters??? okay.
This I completely agree with, Mystique being marketed as human is honestly baffling. Her being human in the movie for a good reason is one thing, but in this respect the marketing just screams of pandering.
