No, Adolf Hitler is not "the Man in the High Castle." That title refers to a fictional character in the alternate-history novel by Philip K. Dick (1962) and its later Amazon TV adaptation (2015–2019).
In the story, the Allies lost World War II, and the United States is divided between Nazi Germany and Imperial Japan. "The Man in the High Castle" is the nickname given to a mysterious, reclusive figure who is believed to have written (or, in the show, produced) works depicting an alternate reality — one where the Allies actually won the war. This character is never Hitler; in the novel he is a writer named Hawthorne Abendsen, and in the TV series the role and mythology around the "high castle" figure are reimagined and expanded, eventually tied to characters who possess or create films/newsreels showing other possible timelines.
Hitler does appear as a character within the story's world — he is depicted as an aging, ailing Nazi leader whose declining health creates a power struggle among Nazi officials, since he is not the man behind the high castle. He is a background political figure influencing the plot, not the enigmatic figure the title refers to.
The confusion likely arises because the story is so steeped in Nazi imagery and alternate WWII history that people assume the mysterious title character must be Hitler himself, but the narrative specifically centers on someone outside the main Axis power structure who possesses forbidden knowledge or media about a different outcome of the war. That contrast — an ordinary but insightful outsider versus the actual dictator — is part of the story's thematic point about truth, propaganda, and alternate realities.