Was a TV show you like ever ruined by the addition of a particular character to the point that you had to quit, or nearly quit?

Scrappy-{dog}-do{ody}

That's gotta be the original bad addition right?

Hasn't been seen (by me, hopefully not by anyone else) since.

“Scrappy Doo” is what we call any new annoying character that threatens to upend the show. “Rose” from Downton Abbey when she was first on.
That's a very old but quite accurate memory. Couldn't stand the runt.
My friends (at the time) called Scrappy "the Yoko Ono of Scooby Doo," and I thought that was apt.

At first, I was very, very cynical about Seven of Nine on Voyager. A tall, stacked blonde in a skin-tight bodysuit? Really? I thought it was just insulting pandering.

I was so annoyed that I would have given up on the show if not for the writers. She was a unique character with tremendous potential, and they took advantage of it. She had the acting ability to pull it off, too. She wasn’t simply there to coast by on her looks, and her character ending up adding value to the storyline. I’m glad I gave her a chance.

Seven of Nine started off as the Born Sexy Yesterday but became awesome. Voyager really had some great arcs
The episode where she's playing as the Doctor really shows her amazing acting skills.

https://memory-alpha.fandom.com/wiki/Body_and_Soul_(episode)

My first experience with this was as a kid, with the Fairly Odd Parents. Their baby (Poof) was just not fun. I felt betrayed and confused that a show I liked could go from my favorite thing to just plain bad lol

I know there have been others, but all my brain can think of is shows that I stopped watching because they got rid of a character. Fear the Walking Dead was alright, the plot wasn’t great but I really loved Nick’s character. Then they ruined it, because of course they did

Scrappy Doo.

I stopped watching Big Bang theory after Bernadette became a regular. The way she constantly belittled her husband and talked down to him really rubbed me the wrong way. Made any scene she was in hard to enjoy until I just stopped watching entirely.

Idk, that show wasn't easy to watch to begin with (the wife and I lived with her parents who would put it on in the background), they were pretty much all mean to each other, everything was mean about that show, everyone talked down to everyone else. I'm not sure when the character you brought up came in, but I think that was the modua operandi of that show.
For me, it was the addition of Amy Farrah Fowler. Every scene she was in it looked like she was trying so hard and nothing she ever said was remotely funny. I actually liked the early seasons when it was a bunch of socially inept nerds navigating life.

Then they started banging 10's and it was Two and A Half Men with Star Wars references.
I only recently binged Big Bang Theory and the only way to watch it is 2x speed. I don't know why but watching it normal speed, the show just seems so stilted and terrible. But 2x speed, the jokes land better and I find the show actually watchable.
It's a character on a comedy show, I had no problem with that. The problem was that Howie's character was destroyed and he stopped being funny after they got together.

The Office. I stopped watching after Michael left and have only just started going back to finish it. It's kinda weird/cool still having "new" Office episodes, even if they aren't the best ones.

I have had the reverse. I watched supergirl pretty much because it had the martian manhunter.

Dr Who and Jodie Whitaker. Dr Who takes charge and tells people what to do. Jodie put everything up for discussion and let the group decide.

Great actor. Lousy writing and direction. I really feel for her.
one thing that got me was I felt they threw a bunch of companions at her because they felt she could not carry the show or something.
For me it was Clara Oswold. Fucking awful character. Capaldi is a great actor and was let down by bad writing over all, but Clara was the absolutely worst thing about his run

It was two characters, Greg and Tamara in Once Upon a Time. The show is campy, but it's still really entertaining. When those two showed up it went from campy, to very stupid. Idk how you can make a couple of characters in a fairy tale story seem completely unbelievable, when you're able to pull off the whole fairy tale to begin with, but that's what happened. I shut several episodes off as soon as those two popped up on screen. S3 Spoilers below:

I was super glad when they were finally killed.

Game of thrones and Euron Greyjoy. The show was declining anyway, but he was unbearable.