Akira Toriyama's Dragon Ball boasts some of the most powerful anime characters of all time. From the mighty Saiyans to the show's excellent cast of villains, it can sometimes seem as though power levels in the series are constantly increasing, but this isn't always the case. In fact, sometimes, certain Dragon Ball characters have actually been known to get weaker over time.
At various points over the past few decades, some Dragon Ball characters have had their power levels nerfed. Sometimes, this is done for the purpose of advancing the story, while on other occasions it seems to be done mainly for comedic effect. Whatever the reason though, these Dragon Ball characters all became weaker at one point or another, albeit temporarily in most cases.
Gohan
There was once a time when series creator Akira Toriyama wanted Gohan to replace Goku as the lead of Dragon Ball, as evidenced by the former's prominent role during the Cell Games saga. However, fans just never really took to Gohan in the same way that they did Goku, leading to Toriyama rethinking his plans during the Majin Buu saga; at the beginning of which Gohan has grown noticeably weaker than he was when he fought Cell.
There's probably some old adage about people growing soft and weak during times of peace which explains Gohan getting nerfed, and this would certainly make sense given the lack of conflict during the seven years that separated the Cell and Buu sagas. Despite opting to walk the path of an academic rather than the path of a warrior though, Gohan does have his true power awakened during the Buu saga, and, eventually, far surpasses his childhood power levels during Dragon Ball Super: Super Hero.
Majin Buu
Like Gohan, there was a point during Dragon Ball Z when Majin Buu was the show's strongest character. He eventually comes unstuck though; both figuratively and literally, with the character being split into good and evil forms toward the end of the series. The latter is defeated by Goku's Universal Spirit Bomb, but Good Buu remains on Earth and decides to live out his days with Mr. Satan at Satan House in Satan City.
By the time Dragon Ball Super rolls around, however, Buu has become a bit of a joke, with his eating and sleeping serving as the punchline to many of the series' jokes. At one point, it does look like he's ready to take things seriously again as he trains himself up to a level above that of even Goku's, but he once again falls asleep and misses out on the Tournament of Power. He does help out with the fight against Moro in the Dragon Ball Super manga, but offers very little during the early stages of the series.
Krillin
When Krillin is first introduced during the original run of Dragon Ball, he's shown to be almost as strong as Goku. In any other setting, two similarly aged people having similar power levels would likely come as no surprise, but, given Goku's Saiyan heritage, it's actually an incredibly impressive achievement on Krillin's part. It's one that Tien can lay claim to as well, but neither he nor Krillin can come close to keeping up in the long run.
Some will argue that it's more a case of the people around them growing stronger while their power levels simply stagnate, and this is certainly true to a certain degree. However, in the case of Krillin, he completely drops off after his death on Namek, with Tien at least offering some resistance in the fight against Cell. Krillin does get his redemption eventually though, with the chrome-domed hero managing to eliminate Majora from the Tournament of Power with a little help from his dirty shoe.
Hit
The Universe 6 saga of Dragon Ball Super introduced plenty of powerful new characters, but none more so than Hit. The interdimensional assassin was able to go toe-to-toe with Goku despite being restrained by the tournament's rules on lethal force, suggesting that his true power went far beyond that of Super Saiyan Blue. In fact, viewers already knew this to be true as Goku was forced to use Kaio-Ken together with the Super Saiyan Blue transformation during the pair's showdown.
Fast forward just a little to the Tournament of Power though, and Hit's power levels seemed to have taken a massive hit, with the legendary hitman initially struggling to keep up with Dyspo. Granted, he is eventually able to get the better of the supersonic rabbit and even gives Jiren a run for his money a little later on, but this remains yet another example of a Dragon Ball character being nerfed for the sake of creating tension and suspense.
Goku
Despite his many years at the top of the Dragon Ball pile, Goku is no more immune to having his power levels nerfed than anyone else. However, unlike with other characters, Goku becoming weaker is more down to Dragon Ball GT not being considered canon, with the Super Saiyan 4 transformation that he attains during the much-maligned Z successor effectively being wiped from existence when Dragon Ball Super became a thing.
It's worth noting that Goku achieves Super Saiyan God form very early on in Dragon Ball Super and so his power levels aren't nerfed for all that long. Even so, upon its creation, Super Saiyan 4 was clearly intended to be more powerful than Super Saiyan 3, which is Goku's most powerful form when Beerus first arrives on planet Earth. Had he truly been able to master the Super Saiyan 4 transformation in GT, the drop off in Vegeta's power levels would have been even greater, as the Saiyan Prince never actually obtained the Super Saiyan 3 form in either the anime or the manga.