Loudmouthed describes a person who talks too much and says rude, hurtful, or foolish things. It is the adjective that matches the noun loudmouth. So we say a loudmouthed neighbour, a loudmouthed fan, loudmouthed comments. The meaning is the same social wound: too much talk, often too loud, and not careful. It is informal, and it judges the person. Though actually, like loudmouth, it is not only about volume. A quiet but careless talker can still be called loudmouthed if the words themselves are ugly or stupid. It is not a kind classroom word.

大嘴巴的（形容愛講、講得又大聲又不得體）