Badmouthing anyone during any point of a job application will certainly make you come across as a negative, critical and with a bad attitude. You wouldn’t want to show a potential employer that you’re someone that may be difficult to get along with.
Occasionally, an interviewer may attempt to test your attitude by asking questions like “Have you ever worked with someone you didn’t like?” This might seem to be encouraging a negative response, but instead of badmouthing a past employer or co-worker, it would be nice to focus on what the problem was and how you had handled the problem. If you recall, draw the focus towards the positive outcome based on a bad scenario.
Stay with the facts and how you contributed to the resolution can be a great way to showcase your soft skills.