There are all sorts of situations in work life that are problematic.

'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.