People have wrongly assumed that Captain Marvel is the strongest character in the Marvel Cinematic Universe, but people tend to give the title of "strongest MCU character" to the most exciting character from the latest movie. They might think Thor is stronger or Thanos is stronger. They're all, of course, wrong. Captain Marvel, in regards to raw, unaided power, is stronger than both of these characters, or, at the very least, on par with them.
But one often neglected fact is that there is a character already introduced in the MCU that far outclasses every other character to have ever appeared in the series. By a lot. There is a character so titanic in strength that no one has come close to bruising them. The most the heroes have done is annoy him into leaving.
And this character will return now that the Time Stone has been destroyed. Captain Marvel might be powerful, but she has no chance against Dormammu.
The Universal Destroyer
The world of Doctor Strange features reality-bending humans who can distort and manipulate existence with ease. The Ancient One effortlessly defeated the Hulk with just a single strike. Doctor Strange could match Thanos with the Infinity Gauntlet for a good while before Thanos ultimately bests him.
And yet none of these characters, with powers equipped to handle Dormammu, could stop the Dark Lord. They could only delay him for awhile.
Dormammu exists as an overlord of an entire universe. He regularly invades and corrupts universes with his dark energies, stripping the dimensions of reality away until all is as he is: timeless, eternal and beyond comprehension. Dormammu exists as a Lovecraftian horror. It is impossible to comprehend the full power of this entity.
Characters like Captain Marvel are powerful, yes, but what chance do they stand against something that exists beyond the material world?
Dormammu Will Return
What evidence, though, do we have that Dormammu will return?
If you remember Doctor Strange, Strange managed to defeat Dormammu not by fighting, but by introducing an unknown element into Dormammu's universe: time. In a rather brilliant sequence, Strange managed to convince Dormammu to leave Earth, or else he'd be trapped in an endless loop where Strange would let Dormammu kill him again and again.
Again, Strange didn't defeat Dormammu in a battle, because Strange knew he'd stand no chance against Dormammu. Which means, now that the Time Stone is gone, nothing will keep this Universal Conqueror away from the MCU. After all, the one thing that could annoy him into surrender is gone. And what does the Dread Lord Dormammu care about honoring promises with a human?
So, yes, Dormammu will almost certainly turn his sights back to the mortal plane. But how does Captain Marvel factor into this?
Captain Marvel is Ill-Equipped
Captain Marvel is undoubtedly one of the strongest Avengers. Sure, Thor can cut down spaceships with Stormbreaker, but Captain Marvel did that with her bare fists. She's incredibly powerful. On a cosmic scale.
However, that said, she solves most of her problems with power and diplomacy. She is diplomatic when helping the Skrulls home, but every other problem so far she has dealt with by punching it really, really hard in the face. This works for physical forces opposing her, but when it comes to something more beyond the material realm? Different story.
Carol Danvers cannot punch Dormammu into submission, nor is she equipped enough to reason with him. She can, of course, beat down his minions. No doubt she'd be able to take someone like Kaecilius down, but to handle this force of unrelenting malignance? Again, different story.
To handle Dormammu, Earth's Mightiest Heroes would need to become increasingly creative. There is the possibility that Dormammu's threat will be contained to Strange's universe and that Strange, rather than overpower Dormammu, will be forced to conceive of a creative solution. But that still establishes Dormammu as an unstoppable force of power.
Dormammu is the strongest character in the MCU, bar none. There is no one who comes remotely close. So, if the heroes of the MCU want to stop him, they'll need to pool their skills together to overcome this unstoppable obstacle.