Post-World War II literature on both sides of the Atlantic displayed much disillusionment, guilt, and pessimism, not the uplighting attitudes of Romantics. Such works as Aldous Huxley's Brave New World. George Orwell's essays and novels such as 1984 and Animal Farm were certainly not optimisitic. Also, during the twentieth century there was much satire written. (Joseph Heller's Catch-22, The Great Gatsby) and, certainly, a great deal of social criticism and protest literature. (James Baldwin's Go Tell It On the Mountain) These two genres of satire and protest aare definitely not Romantic.
Are you referring to 20th century world literature or literature from a specific country?
If one applies your statement to American Literature, I would disagree with it because Romanticism is idealistic, includes many supernatural elements, and idolizes nature. In contrast, Modern American Literature (1900-1950) often includes disillusionment (especially with the American Dream), realistic elements, and harsh life in the city or in the country (Of Mice and Men, The Great Gatsby). I would argue that Realism is closer to 20th Century literature.