Mewtwo is completely outclassed by calyrex shadow in almost every way.
Mewtwo doesn't get better/ gets better depending on what you're comparing.
Better special attack, better ability, more speed you name it. Even after getting nasty plot, it isn't useful as calyrex shadow has also got nasty plot. Moreover, every good team has yveltal which will K.O. you with knock off/Sucker punch/Dark pulse if no nasty plot is up or your luck didn't work for 56% chance to OHKO after +2. Finally almost all Zacian use crunch which means your sweep will be stopped with ease. Personally you can still try it as you may just chip yveltal and then your calyrex shadow will sweep.
Is it better:
Last gen, that is USM ubers, mega Mewtwo Y was rank A- but Mewtwo in this gen is B- because it already had a huge special attack. Yet if you mean normal Mewtwo of gen 7 ubers, yes it did get better.
Hope I helped!