Or are dealing with a genuinely painful day-to-day work situation and are struggling to keep a lid on it.
I get the point that venting about the problems of one's current or previous environments is bad form, and not anything anyone really wants to hear about it. But the touch of schadenfreude in your characterization of the (often very real and painful) situations these people are helping doesn't exactly help, either.
You can certainly voice displeasure with your current working arrangement tactfully (who's going to be able to verify what you have to say? nobody) but if you can't keep yourself emotionally under control which asked an open ended, indirect, and predictable question you're throwing out all sorts of warning signs that you should not be hired.
'Badmouthing' one's former employer would more likely just indicate a general lack of tact, and it wouldn't necessarily even say that much about the current employment situation.
Not having a good fit in your current situation is just fine, you just have to communicate it thoughtfully to prospective employers, to not be cynical or petty about it, and not belabour the issue or let it dour the overall disposition of your candidacy.
A recruiter is not your therapist. Don't treat them like one.
The point there's no need to feel smug about people who are dealing with seriously nasty stuff.
I won’t bad mouth peers but I will be honest if certain processes or practices really bother me. But I also raise those concerns as an employee too.
Would I say “yeah I work with idiots”? Absolutely not. But I have and will continue to be honest about pain points in a respectful manner. Especially when they are disagreements about things like priorities. I like to let enough of my personality come through during interviews that people know what they are getting. Even if I disagree with you I’ll approach it from an angle other than “you’re wrong”. I care about making things better for people a bit more than I care for money. And I believe we all have a responsibility to leave the world better than it is now.
If companies or recruiters see those as red flags I’m ok not moving forward. Same reason I typically don’t hide that I have weird colored hair and tattoos.