Downtrodden people are treated badly, and without respect, by others who have power over them. The picture in the word is of someone being stepped on. You use it for a group, or for a long unfair situation — poor workers, people with no rights — more than for one bad day. It is a serious, slightly bookish word; news, speeches, and stories use it. You would not usually call a classmate downtrodden because they lost a game. Kind talk still tells the messy truth: this word is about power, not about being a little sad.