Buggy does not need to be strong. I keep seeing people trying to justify Buggy becoming a Yonko by saying that he has either been hiding his power this whole time or that he must have awakened during the timeskip and now has some broken ability. Both of these theories are unnecessary and don’t really make sense (from a narrative standpoint, I think). There isn’t a strict criteria for being named a Yonko, but I think that a captain’s power is only the third most important factor.
Firstly, the Yonko are Emperors of the sea. At the very least we can assume they are able to stretch their influence over a particular area. Big Mom ruled over Whole Cake Island and the surrounding islands – including Fishman Island. Kaido obviously ruled over Wano with an iron fist, but we know his influence stretched as far as Dressrosa (and Punk Hazard?) through Doflamingo. Blackbeard took over all of Whitebeard’s territory during the timeskip (with the exception of Sphinx Island, which is still under Marco’s protection, and Fishman Island which went to Big Mom and now Luffy). We’re not sure how far Shanks exerts his power, but he could possibly claim Elbaf and a few smaller islands, like the one the Barto Club went to, as his territory.
Now admittedly, all of these characters are incredibly strong. They’re among the most powerful in the series, but that’s not necessarily why they are Yonko. I would argue that the maintenance of their spheres of influence is far more important to being a Yonko than strength. There are other characters on their level who were never named Yonko for various reasons. Mihawk isn’t interested in being a pirate and only became a Warlord to get the Marines off his back. If he wanted he could certainly become a Yonko. By the time Roger died Rayleigh could have easily taken over and was strong enough to be considered a Yonko as well. Dragon is the most wanted man in the world, but he’s not a pirate so he’s not a Yonko. We don’t even know if he’s powerful (I assume he is given his background, but crazier things have happened lol).
I think their threat level to the World Government is the next most important factor to being a Yonko. Big Mom and Kaido were both actively searching for the Poneglyphs and trying to find the One Piece. They were both close to unraveling the mysteries of the Void Century. Whitebeard wasn’t trying to become King of the Pirates or find the One Piece by the time he died, but he was still considered to closest to it – meaning that he was also a huge threat. All 3 also commanded massive fleets with dozens of crew members with Haki and devil fruits. I don’t think Shanks’ goal is the be Pirate King. He probably wants to make it to Laugh Tale, but I think that’s more of a way to honor his mentor than anything else (I certainly could be wrong about that). BUT he was a former member of the Roger Pirates so the World Government is definitely worried about what he might know. I’m sure you can guess where I’m going with this – Buggy was also a member of Roger’s crew. That makes him dangerous.
So now, why is Buggy a Yonko? His notoriety/infamy. He currently controls the Underground. He’s an ex-Warlord and the Captain of a large crew. He is a former apprentice of the Pirate King himself and comrade of Shanks, who is now the longest-tenured Yonko. He escaped Impel Down along with another new Yonko in Luffy and another ex-Warlord in Crocodile. His feats match up with the most powerful pirates in the world. Buggy’s rise to power has always been about perception. He doesn’t need to be strong himself because people just think that he is. Who wouldn’t? Look at what he’s “accomplished”! He must be strong!
Other than Luffy beating him in Orange Town, no one has ever seen him lose a fight. Even though he’s technically got toe-to-toe with the likes of Mihawk and Magellan. It is also entirely possible that some of the ex-Warlords have joined forces with him. Or that he found the Ancient Weapon Uranus.
It is from Captain John’s treasure mark that Luffy gave him in Impel Down arc. The map shows the location of the treasure, and the treasure could turn out to be Uranus. Maybe Buggy accidentally found the Ancient Weapon while looking for treasure, by sheer luck, which is the ongoing theme for Buggy’s character.