Avengers: Endgame confirmed what so many had predicted since Age of Ultron: Captain America is worthy of wielding Thor's enchanted hammer, Mjolnir. But, as memorable as this moment was, it may have come at a cost to Steve Rogers. The 2019 sourcebook The Wakanda Files: A Technological Exploration of the Avengers and Beyond shed new light on the technology and weapons of the Marvel Cinematic Universe, including Mjolnir and Stormbreaker.
As fans already know, only a select few are worthy to wield Mjolnir due to the spell placed on it by Odin. But even without the spell, it seems Asgardian weapons simply aren't meant to be used by mortal beings. The Wakanda Files states the following:
There is a psychic or telekinetic connection through these weapons that magnify [Thor's] abilities. This might be why Thor has said on more than one occasion that if one of us were to wield Mjolnir, there is a chance that even holding the weapon would crumble our minds into madness. The psychical effects of the weapon on a mere mortal could be catastrophic.
Thor's Appearance in Avengers: Infinity War Delves Deep Into Asgardian Weaponry
Thor said something similar to the Guardians of the Galaxy in Avengers: Infinity War. After being rescued by the Guardians, Thor tells them he has to go to Nidavellir to get a weapon strong enough to kill Thanos. When Peter Quill suggests they should all get such weapons, Thor claims they aren't strong enough and that "[their] bodies would crumble as [their] minds collapsed into madness." Assuming Thor was telling the truth, that implies mortal beings are physically and mentally incapable of utilizing Asgardian weapons like Mjolnir and Stormbreaker.
Even with these parameters, it's difficult to say what effect the hammer actually had on Cap's mind. After all, he may be mortal, but he's still much more than that. Cap's powers come from Dr. Erskine's Super Soldier Serum, a chemical solution meant to enhance the human mind and body. The serum greatly improved his physical strength, mental capacity and ability to heal, among other things. These powers may have made him less susceptible to the effects that Mjolnir would normally have on a mortal.
When Captain America wielded Mjolnir, however, he didn't crumble as Thor described. Instead, Mjolnir appeared to enhance his abilities and even allowed him to summon lightning. Cap was also able to call the weapon to him, so, presumably, he did share the psychic or telekinetic connection with Mjolnir that was described in the book. He even used Stormbreaker briefly before Thor insisted they swap weapons. While Cap didn't seem to be thinking or acting much differently while wielding either Mjolnir or Stormbreaker, that doesn't necessarily rule out long-term effects on his mind.
Captain America Has a History of Proving He's Not an Average Mortal
This wouldn't be the first time Cap has withstood something meant specifically for Asgardians. In Age of Ultron, Thor shared with Steve some Asgardian ale, which was not meant to be consumed by mortals. They also shared it with an insistent World War II veteran, played by the late Stan Lee. While the vet became intoxicated, Steve's enhanced metabolism and regenerative cells ensured he would be fine. Despite Thor's own warning, he didn't hesitate to pour the ale into Steve's cup. Similarly, Thor didn't seem concerned when the Super-Soldier picked up Mjolnir but was simply excited for his friend. Cap may technically be human, but Thor also knows he's not an average mortal.
Because Captain America is one of the first known mortals to wield Mjolnir in the MCU, there aren't many cases to compare him with. The other characters who have picked up the weapon are Odin, Hela, and Vision, none of whom are human. In the comics, however, a few others have wielded the hammer, including Eric Masterson, Storm and Jane Foster. While wielding the hammer didn't drive any of them insane, it did have a significant physical effect on some of them. In Jane's case, holding Mjolnir transformed her into an Asgardian and purged all toxins from her body. Unfortunately, this included her chemotherapy drugs, and as a result of wielding the hammer, Jane erased all progress on her cancer treatment. Although the same rules don't always apply in MCU, there may still have been a steep price to pay for any mortal who wielded Mjolnir.
While Mjolnir didn't appear to take a physical toll on Cap, he still could've experienced some psychological effects after the fact. At the end of Endgame, Steve returns to the 1940s and marries Peggy Carter. Years later, he caught up with the present and passed the shield on to Sam Wilson. Having been gone for so long, it's possible that Steve experienced some mental side effects from Mjolnir. Equally, his whereabouts and ultimate fate remain unknown, even after the events of The Falcon and the Winter Soldier. Conversely, the Super Soldier Serum could've alleviated the psychological effects along with the physical. Either way, Cap took on a greater risk than he was aware of when he picked up the hammer.