“In social psychology, a stereotype is a thought that can be adopted about specific types of individuals or certain ways of doing things. These thoughts or beliefs may or may not accurately reflect reality.”
So you can quote from a dictionary. Your example, walking around with a sign, doesn’t reflect a “type of people.” Plus, if I saw a person advertising, with a sign, that they were violent and dangeorus, I’d probably think they weren’t. The stereotype is that people who are violent and dangerous don’t advertise that fact with a sign, not that they DO advertise that fact with a sign.