I feel like Mihawk counts. If only because he hasn't been given anything to do since training Zoro. Other than running away from the Marines and joining Cross Guild so they can protect him. He's still probably one of the strongest people in the world, but he really hasn't gotten a chance to show that off in a LONG time. Causing Mihawk's image to decline. Really, it's only his title that still keeps him in people's mind. He's even lost his goth girlfriend! Mihawk's life has just generally gotten worse since the Timeskip.
CP9 in general, maybe? They went from one of THE strongest groups in the world, to outlaws on the run from the Government, to...just rejoining them again because of a SBS side-story? They went from boss-level enemies, to mini-bosses. I don't know, they haven't fully lost their glory. It's just that the Strawhats have grown so much stronger than they have. They're not the threat they used to be. And they're not even interesting character-wise to compensate for that. They're just minions, now. And that's a shame.
Do we want to count Yosaku and Johnny? They went from recurring characters in the manga, to not even being worth a cameo. I don't know if enough people care about these two enough to really bother listing them. But, there's no doubt they've suffered from obscurity post-timeskip. They went from bounty hunters to fishermen. They have definitely declined.
Similarly...Arlong? I mean, he got defeated. His story is over. But, other major One Piece villains have made reappearances since their defeats. But, not Arlong. When candidates for who could join Cross Guild in the future come up, no one even bothers to mention Arlong, even though he's still alive. Doflamingo and even Weevil are always brought up to join Cross Guild later, but almost never Arlong. Most people just see him as too weak. He arguably went from THE most iconic One Piece villain ever, to not even being worth a cameo. If that's not a drop off, I don't know what is.
Jinbe. Arguably, other Strawhats have fallen further, since their dreams are on the back-burner due to the plot. But, Jinbe never even GOT a dream of his own. I think a lot of fans, pre-timeskip, assumed we'd eventually learn SOMETHING interesting about Jinbe. Something that would make him "worthy" of becoming a Strawhat. But, no, not really. Even Jinbe's flashback doesn't get to be about him. His "backstory" has more to to with Fisher Tiger and Otohime than any tragedy that happened to him. He doesn't even get a iconic, memorable backstory that tells us why he has the dream he does, like every other Strawhat has. Jinbe's just along for the ride, in his entire life. He doesn't really want to be the "World's Greatest Helmsman", or to learn how to pilot every vehicle in the world and become the "World's Greatest Pilot", or something. Jinbe just wants to end racism. Which isn't a personal enough goal to actually give him a personality, in my opinion, since it's not really about him. I'd say Jinbe is the Strawhat that's suffered the most from Oda's Post-Timeskip writing style, because he never even GOT a dream to PUT on the back-burner.
Finally, Monkey D Dragon. His first appearance at Loguetown is so mysterious and intriguing! Then we find out he's the leader of the Revolutionaries and Luffy's dad. So much hype! And then...he does...NOTHING for the next 20 years! The best he's gotten is his underwhelming showing in the God Valley flashback. At this point, he's a joke. He's a punchline in this manga. There ARE ways that a character can do nothing and still build up their hype. I think a lot of people are still DYING to see Enel again, even after not doing anything for years. But, that's not the case with Dragon. This might change in the future. But, for right now, he's the posterboy for "squandered potential". I think Dragon gets my vote, personally. Fans still respect Mihawk. And Arlong's story is over. But, Dragon is a walking punchline that I'm not totally convinced will ever recover his former gravitas.