For me if I had to pick a good contender it would be the UK version of The Office.
I know many tend to debate how Ricky Gervais really fell off and how he repugnantly acts like a whiny centrist edgelord but me personally IMO I actually don’t think he was ever funny not even a little.
His big break through television was just so painful to sit through it’s so charismatically boring the characters are completely generic at best (notably Tim) or straight up insufferably unlikable at worst (especially the protagonist David FUCKING Brent) and most importantly the humour is just embarrassing.
Always seemed like The Thick Of It but without the nuisance tongue in cheek and charming satire.
Just to mention it, this is literally named the Seinfeld is Unfunny effect on TVTropes
I think the bigger issue is that most of the show isn’t that good. Less than half the seasons are good, and the lows from the bad seasons are really low. Watching it on a streaming scenario exposes this a lot more than reruns of the good parts.
I actually still enjoyed it for the most part last time I went through. Jerry’s standup is terrible and I have no clue how he ever got an audience, but the actual show I enjoyed most of, even if nowadays pretty much all the plots would be solved because everyone would have a cell phone.