WARNING: The following contains spoilers for The Falcon and the Winter Soldier Episode 6, "One World, One People," now streaming on Disney+.
It's pretty much a given that fans of the Marvel Cinematic Universe are going to be seeking clues and Easter eggs to fuel theories about how things will be expanded. The Mephisto conspiracies ran rampant during WandaVision and now, with a Secret Invasion series in the works, they're looking for hints regarding who might be a Skrull in disguise. However, despite her heel turn in The Falcon and the Winter Soldier, one person who just can't be a shape-shifting alien is Emily VanCamp's Sharon Carter.
Admittedly, Sharon made a huge change from the Agent 13 audiences knew in Captain America: The Winter Soldier and Civil War. In Falcon and Winter Soldier, she was bitter that America still had her as a fugitive, so Sharon became the Power Broker in Madripoor, developing and selling the Super Soldier Serum to create chaos as a means of revenge against world powers.
Some people think this is the behavior of a Skrull infiltrator, trying to destabilize the planet by turning terrorists into Super Soldiers and leaving the world vulnerable. This could lead to aliens conquering, similar to what Thanos did when he came for the Infinity Stones against a fractured Avengers. This theory has been solidified by Sharon getting pardoned by America and being put into a position to steal research, weapons and secrets to sell in stealthy fashion, per the post-credits of the finale.
All these are pretty solid points to build a case, but the first thing that debunks Sharon being a Skrull is her using the face-swapping mask Black Widow wore in The Winter Soldier. In Falcon and Winter Soldier's finale, Sharon disguised herself with the tech, then revealed the truth to Bucky, all so she could get to Karli and try to bring her back to her side. If Sharon were a shape-shifter, she could have turned into law enforcement, slipped into the Global Repatriation Council zone in New York, then morphed into a Flag Smasher and gotten to Karli. But by using the mask, she proves she's human.
Secondly, when Karli did shoot Sharon in their confrontation over how she stole the serum, Sharon's blood was red. If she were an alien, this wouldn't be the case as Skrull blood is darker and more purplish in color. Also, blood problems could've shown up with Sharon's seeking treatment to bandage up her wound. If she were a Skrull, she'd be risking exposure by doing this.
Lastly, if the Skrull wanted to stay off the radar as the Power Broker, whether it created this identity from scratch or usurped it from someone else, Sharon wasn't the form to take. Before her pardon, she was wanted by Interpol and such, as well as being someone many heroes and villains might recognize. Ultimately, the Skrulls are all about laying low, operating in secrecy and not drawing attention, as their many comic depictions have shown. Thus, using Sharon doesn't make much sense because this could scupper the invasion before it even begins.
Directed by Kari Skogland, The Falcon and the Winter Soldier stars Anthony Mackie, Sebastian Stan, Emily VanCamp, Wyatt Russell, Noah Mills, Carl Lumbly and Daniel Brühl. The series is available on Disney+.