Harmony (Buffy The Vampire Slayer)
Buffy literally laughed at the idea of Harmony being taken as a legitimate threat even if vampires are fairly powerful creatures in the Buffyverse. Harmony was completely crippled by her desperation for the other characters to like her so much that she lost all credibility as a force to be reckoned with.
Marcus Pierce (Lucifer)
The burden of his immortality must have chipped away at his desire to think things through because that is the only way to explain Pierce betting everything on a love scam that put him in Lucifer's cross-hairs. He has spent thousands of years of wallowing in his existential crisis, but it took him becoming mortal (something he's wanted for millennia) for him to decide he didn't want to die after all.
Jimmy's Clients (Better Call Saul)
At this point, no one can blame Jimmy for going bad and becoming a meth kingpin's go-to man. Representing bad guys who can't be bothered to do evil things the right way can make even the most resilient of us to snap.
Tobias Whale (Black Lightning)
Tobias is a pretty scary dude on the surface. He's unnaturally strong, he doesn't age, and he runs the gang that has terrorized Freeland for years. Then you see all the losses he has accrued since he resurfaced: Syonide, Tori, even Khalil is now a loose cannon that can be aimed straight at him. He might very well prove us all wrong, but so far he has been mostly bark with very little bite.
Ambassador Dred (The Outpost)
He sent mercenaries to carry out his genocidal wishes on Talon's people only for them to let one slip through their fingers, a fact he learned of years later. But what does he do again? hire another mercenary to find and destroy the book of names which she failed to do. When he finally got around to handling things himself, he got a hot poker to the face and barely made it out with his life.
Bulshar (Wynonna Earp)
Another overhyped demon who disappointed, not only in how little he managed to achieve but also how he went out with nothing more than a whimper, not to mention how the whole story got bogged down by his overly complicated plans.