I would say Koulibaly's peak was during 2015-2019. During that time he was a real top defender and was rated as one of the best stoppers in the world. He was rated by Italian media as the best center-back during 2015-2020. I can say that his Napoli days were a lot faster both in terms of top speed and acceleration, which was one of the important parts of his game. His marvelous and outstanding top speed made him fit well against top wide forwards like Salah and Mbappe, which a lot of fellas had already mentioned. He was also known for his durability during his peak, he rarely missed the games during 2015-2019.
During his peak, his impact with Napoli was simply almost exceptional. As you can simply notice from Napoli's performances in Calcio. In 2015-2019, Napoli's ranking in Calcio was never below third place, and they got second place three times. You could say that Napoli's attacking unit was excellent and it was a major part of Napoli's performances during that time. However, Koulibaly's impact during that time couldn't be overlooked for sure.
Napoli's points in Serie A during 2015-2019
|3rd place|| Napoli( 2016/2017)||38||26||8||4||94||39||+55||86|
|2nd place||Napoli( 2018/2019)||38||24||7||7||74||36||+38||79|
Post 2019-present, Koulibaly's situation has become different from his peak period. He has become a bit injury prone as I posted during 3-4 months about his injury records. After 2019, he never competed in league games for over 30 matches( 2019/2020, 2020/2021, 2021/2022). He could also be noticed his declining in his physical attributes like top speed, acceleration, or even agility. As you could say that Napoli 2019/2020 faced a lot of problems from many parts. But you couldn't deny that he had a lot of poor performances in that season too. His decline affected Napoli directly, especially in terms of points and performance( 2019/2020 and 2020/2021). He started to play well again in the 2021/2022 season and received the player of the month in September 2021.
You must accept that he is a very aggressive defender who could be clumsy and make reckless tackles a lot of times. His biggest strengths were athleticism, physicality, and tackle. He is also a person who performs best as a pure stopper. One of Napoli's legendary defenders( Ruud Krol) once told that he couldn't be a leader of the defensive line. He also criticized Koulibaly a lot of times.
He was already lost some of his athleticism before he joined Chelsea and athleticism was the main part of his games during his peak. For his clumsiness, it is a natural part of his games, and probably a lot of stoppers. He usually makes a lot of clumsy tackles and challenges. Yes, he needs to adapt in English football but I am also not sure whether he will have a successful career at Chelsea or not. As he already lost his best abilities atm. If Koulibaly had wanted to move to English football, he should have moved during 2015-2019 when his athletic abilities were still top-notch. In fact, he wanted to stay at Napoli until the end of his career.
You can see one thing, he never receives any personal award outside Serie A Player of the Month in September 2021 since 2019-. And that could tell all of you guys a lot of things about his general performances. He is already past the peak of his career.