You call something a travesty when it wears the name of a good thing but is actually a terrible, insulting version of it. That is the whole feeling of the word: the false costume. A trial where the judge has already decided is 'a travesty of justice' — it is called justice, it looks like justice, and it is the opposite. Notice that the word almost always comes with 'of': a travesty of justice, of democracy, of the truth, of the original book. Without the 'of', people still use it alone to mean 'a disgraceful thing' — 'It's a travesty that she wasn't chosen' — and that use is common and a bit loose, more emotion than argument. It is strong and formal-sounding, so it lands hard in the news, in speeches, and in angry reviews. There is also an older, narrower meaning: a piece of writing or acting that copies a serious work in a silly way, to mock it. You mostly meet that one in books about literature. And there is a verb, to travesty something, which means to make that mocking copy — quite rare now.