During the First World War and until 1922, yes, and openly so. After that Mann began to defend the republic and, in the 1930s, became one of the best-known opponents of Nazism. The honest answer is that he was conservative for a phase and then changed position, without fully renouncing what he had written.
At the start of the war, Mann wrote that the conflict was felt as purification and liberation. In Reflections of a Nonpolitical Man (1918), a book of 500 to 600 pages, depending on the source, he defended German "culture" against Western "civilization," treated democratization as foreign to the German nature, and argued with his brother Heinrich, who held a more liberal position.
The public turn came in 1922. After the assassination of Foreign Minister Walther Rathenau by right-wing extremists, on June 24, Mann delivered the lecture "On the German Republic," asking intellectuals to support the Weimar Republic. The Portuguese-language Wikipedia describes him as a "republican of reason" (Vernunftrepublikaner). On October 17, 1930, in Berlin, he gave the "Appeal to Reason," in which he condemned Nazism. In 1933 he left Germany and never again lived there, and from October 1940 onward he spoke to Germans in monthly BBC broadcasts. The Konrad Adenauer Foundation summarizes the trajectory: he became the most important defender of democracy among German writers and, in the United States, gave up the last doubts about his earlier position. The Magic Mountain was written in this interval, between 1912 and 1924, and Wikipedia notes that Mann's ideas about politics and humanism changed while he was writing it.
The turn was not a clean break. According to the English-language Wikipedia, Mann never fully disavowed the 1918 book, which he saw as an important stage in his formation, and the book was not translated into English while he lived, which, according to a scholar cited by the English-language Wikipedia, was intentional.
On Clavis: The Magic Mountain.