Saying something negative isn't inherently toxic. But saying something negative in a job interview is walking a line. Everyone everywhere has experienced negative factors in a prior job, so it's not exactly a revelation if someone has some war stories.
But what a person chooses to focus on does say something about how that person thinks.
If I ask someone what they dislike about their previous job, and they say something like "there were times when management would change directions at the last minute and cause the whole team to scramble", that's relatable and not necessarily a red flag.
If someone starts venting about low quality coworkers and shitty management, that's probably a red flag.
If someone volunteers negativity unprompted, that's probably a red flag.
My point here is that discerning between toxicity and honesty is usually possible, and what a person chooses to be negative about is a signal that helps tell the difference.
What I don't want on my team is a culture of negativity. A negative/pessimistic default is a wet blanket that shuts things down before they have a chance to get started. It creates tension where it need not exist. And it requires significant effort to counteract once it exists on the team. And to reiterate, I'm not looking for toxic positivity either. That's a separate problem.