I think "pet phrase" would work. "Catch phrase" is probably also okay, but I think I would understand "pet phrase" more easily. I wouldn't say "mantra" in this case -- a "mantra" is usually some kind of personal ideal, goal, or belief that a person repeats as a deliberate reminder (it could be something religious, political, related to improving the person's own self-image or mental health, etc.).

Sometimes that can be called a 'verbal tic', when someone uses words such as 'like', 'you know', 'and stuff', etc, repeatedly through their conversations , especially when the phrases are totally out of context.

Interesting question. I have no idea...