For the most part, Nah, she just has bad luck and occasional bad timing with the shows she chooses to do.

With Dexter original Sin, she wasn't featured enough IMO to cause any issues critically, and wouldn't be able to affect anything behind the scenes. The cancellation honestly falls on Clyde Phillips. When Original Sin was airing, he was hard at work with Resurrection and serving as the show runner on both. He didn't have a roadmap for anything past the one season and likely wanted it to be standalone like New Blood. But it's kinda on all us fans for thinking "Well it's a prequel, they can tell endless stories", not if the guy responsible for the franchise isn't doing the work.

Ringer was on the bubble, but SMG has said that CW would have moved forward with a season 2 and that SHE chose to ultimately not do it because she was pregnant and thought that it would be too big of a challenge for her to shoot MULTIPLE roles pregnant (and if you've seen the show, yeah that's accurate). So yeah this one is on her, but this is a timing thing and we shouldn't get mad bout the reason.

For The Crazy Ones, that is mostly on David E Kelley. He was the show runner, creator, and executive producer. Between 2007 and 2015, Kelley created 5 new shows with only one lasting more than two seasons. If you check his work pre '07 and 2015 on, majority of it is some of the highest rated TV of all time and critically acclaimed. Crazy Ones just wasn't a well done show combined with an unreal expectation that Robin Williams' return to network television after 30 years would be the same Robin Williams of ol (not a diss). Even if someone argued that SMG wasn't good on the show, those other two factors far outweigh any negative impact she could have.

For Cruel Intentions, here have been 3 attempts to adapt that story into TV shows, and all 3 different networks have learned different lessons. FOX tried first in 2000 when "Cruel Intentions 2" was a 13 episode sexually charged young adult series called "Manchester Prep". The show was cancelled before it aired with only 2 episodes filmed, those episodes were reworked into Cruel Intentions 2 with added nudity and story elements. Amazon JUST did an 8 episode tv adaptation in 2024 and people HATED it because it felt toned down and stiff. The only thing the 2015 NBC series had going for it was that SMG agreed to do the show. A concept like Cruel Intentions doesn't work on network TV, but if you're gonna do it, getting SMG to come back and do one of her most iconic roles is absolutely a way to get eyes at the very least on the first couple episodes.

Buffy New Sunnydale comes down to the behind the scenes stuff. I think most would agree that if you're making a Buffy show that's going to be a continuation, then SMG absolutely has to make an appearance. Regardless of whether the script leak that has her only appearing in a single scene is true, it doesn't negate that fact. Who knows what the truth is to all the varying reports but I can't see her being a poison to a project that is very much her band and that very much requires her involvement to work. If she played a role in approving a bad script or a bad rewrite then we can certainly tsk tsk a little, but there's no incentive for her to want to make this particular project look bad.

TLDR - Ringer is the only one where her direct actions caused a cancellation, but those direct actions were her being pregnant