When you mean mature i think it's certain folks way of saying more serious because i don't agree that mature equals a more serious take.
For example bvs was more a more overall a darker serious film then civil war and doctor strange but it was not a better film then those two and not any more mature.
In fact others would say doctor strange and civil war was more mature then bvs.
Black panther could be a more serious mcu film then winter Soldier.Then we captain marvel coming too.Of course there have been other mcu films that were more serious to varied degrees in the mcu.
I think upcoming avengers movies will be more serious then past ones since the russos are directing them.
We don't know what other mcu movies we will get in the future by the way.
Of course the current marvel shows are more serious then the mcu movies so far.
Being more mature or serious does not mean films or shows will be better.
The avengers first movie for example i think is better then all the x-men films i have seen.
It is not as serious on average compared to main team x-men films but avengers is still a serious film overall.
(The tone for the first two avengers films is more similar to superman the movie and superman 2)
Note- i have not seen logan or deadpool yet(will later when i have more time) but the rest of the x-men films i have seen.
I still believe mcu films are better then current x-men films on average and are way more fun on average.