Chandler Bing starts off in Friends as the funny, quirky guy who everyone can relate to - he's got a boring job, but he works hard, and he's permanently unlucky in love and awkward with women. Sweater-vests aside, he's a pretty average dude... at first.
As the show goes on, though, Chandler becomes increasingly annoying, rather than funny. His jokes become a joke, he's over the top, and rather than just being unlucky in love, the audience starts to think he might just deserve to be alone, given how he treats his dates. He may be sweet with Monica, and it was great to see him change career, but overall, he just got worse and worse.
The Way He Treats Janice
Janice is the butt of so many jokes by the Friends gang - and for no good reason! She has an irritating laugh, but beyond that, there's really nothing wrong with her - and her relationship with Chandler just shows how he gets worse throughout the show. At first, she's just someone he dated and broke up with, then went back to. Which is pretty reasonable, but things just get worse and worse. He goes back to her time and time again, breaks up with her, treats her badly, and makes fun of her to his friends.
He's Increasingly Homophobic
Friends' rampant issues with toxic masculinity are no secret, but Chandler's homophobic comments go from the odd 'real man' joke (that would have been the norm at the time) to a pretty concerning level of homophobia when it comes to his father (who is trans and performs in a drag cabaret). It's clear that his parents' divorce truly affected him, but he still spends time with his mother, while his father is cut almost entirely out of his life. His 'real man' jokes also seem to get worse as the series progresses, which is never a good look.
Commitment Phobia
Initially, Chandler is just a bit awkward in love - which fits, as he's just not a charmer like Joey. However, as the show continues, this goes from a fairly average level of awkwardness around dating to a full-blown commitment-phobia that even leads to him almost running out on his wedding day. It takes a relatable quirk and turns it into an overblown punchline.
His College Flashbacks
At the start of the series, fans like Chandler... until his college flashbacks reveal that he has been several different flavors of awful over the years.
The way he treats Monica is just awful, and while this wouldn't count as the character getting worse in a chronological sense, the fact that the fans only find out about it as the show goes on means that it contributes to the issue.
The Way He Treats Women
Janice isn't the only one! In the early seasons, Chandler is a bit hopeless with dating, but generally well-intentioned. In later seasons, though, this translates to being wildly inappropriate, and even a bit of a player. His storyline with Rachel's boss, and the reveal that he promises to call his dates when he has no interest in seeing them again, just doesn't suit him. He worked so much better as a sincere screw-up with women, because everyone could feel for him about it.
He Gets Increasingly Jealous
Jealousy is never a good look, and as Friends continued, Chandler became more and more jealous. In the early seasons, he seemed much more open and reasonable about relationships - he laughed about Ross's jealousy over a sex dream Rachel had, and he even dated a polyamorous woman and was happy (for a while) to be one of her multiple partners. However, as the show goes on, he gets more and more freaked out about Monica finding other men attractive, flirting, or even just seeing an ex of hers.
His Complaining
At first, it's kinda cute - Chandler, like the rest of them, just can't catch a break. However, as the show goes on, it becomes clear that Chandler was actually raised pretty darn rich (there was a butler and a pool boy in his house!), that his family loves him, that he has one good job (that he then leaves for a new career that he loves), that he's financially stable, and that he is seemingly routinely dating gorgeous women... yet he's complaining about it all. Constantly.
His Relationship With Joey
Chandler's friendship with Joey may be one of the closest in the series - but even this deteriorated as the show continued. He starts patronizing Joey, treating him like an idiot, and trying to trick him into taking financial help. He slowly starts to treat Joey more like a child than a friend and equal, and it's not fun to watch this friendship become so imbalanced.
The Way He Treats His Other Friends
Joey isn't the only friend that Chandler doesn't exactly treat well. His jokes get pretty brutal, but it's also surprising to see his relationship with Ross, his college roommate and the reason he's part of the gang in the first place, be revealed to be less than perfect. Chandler tries to trick Ross out of the house when he moves in with them, he lies to Joey and makes fun of him (and his admittedly tacky gifts), and then he starts sleeping with his best friend's sister and hides it from him. Yikes.
His Jokes Just Stop Being Funny
Chandler is known as the joker of the bunch (so much so that when Monica describes someone else as the 'funniest guy', Chandler throws a fit over it), but as the show goes on, his jokes shift from self-deprecating to more pointed - and more at his friends' expenses. While he does continue to put himself down (which also gets old), he also starts to make fun of the people around him. They may laugh, but it's not a great look for someone who is meant to be a great friend.