And before you ask... "How would mystique know what Natasha looks and sounds like?"
Sooo... I'll try to be brief... A lot of canonical information and, in the end, an opinion.
To start... Mystique wouldn't need Natasha alive to pull it off. I think your doubt stems from believing she can only mimic living individuals... because she has abilities related to biology... BUT no...
Canonically, Mystique has undergone numerous enhancements... all to allow her to achieve more and more capabilities... okay... And how would she manage to mimic Natasha Romanoff?
Nat spent years working with major orgs like SHIELD, Avengers, etc, so there's a ton of data on her... mission footage, voice recordings, biometric scans, eyewitness accounts.
Mystique is exactly the kind of person who knows how to get that kind of intel. In addition to her enhancements, she has a personality that truly leans towards anti-heroism... What makes a person truly capable of anything...
From this it's just her skillset doing the heavy lifting. As we know she can perfectly replicate physical traits (face, voice, even retinal patterns), and she's trained enough to study posture, speech rhythm, and those tiny behavioral quirks people don't consciously notice. Yeah... Mystique is basically the ultimate spy plus her method actress rolled into one. That's the main point here... it's less "she magically knows Natasha" and more "she builds a near perfect reconstruction from intel plus experience."
The only thing she can't copy is Natasha's actual memories... IMO she can't glean certain personality traits because Natasha was just as skilled as she was at the game of deception... i think someone who knew Natasha intimately would easily discover the truth.
But everything else is fair game. It's perfectly normal and acceptable for her to mimic Natasha.
I hope this tweet finds you smarter than when i found you.