If someone is mild-mannered, they are gentle and polite. They do not shout, they do not push, and they do not pick fights. Mild here means not fierce or harsh; the second half comes from manners, the way you behave with other people. English often uses it of people who look quiet on the outside — a teacher, an office worker, the neighbour who always says please. Comic books made a set phrase famous: a mild-mannered reporter, for a quiet person who has a surprising other side. It is a kind description, not an insult. Though actually, it can also sound a little as if you are saying this person seems harmless, almost too quiet to be dangerous. It is an adjective, and it keeps the hyphen.