No, Ciara is not transgendered. According to Ciaras early life
she was born as the only child to her
parents, and while it does not state that she is not transgendered(which is likely because wikipedia sources only factual information), the article uses female pronouns to address her, and makes no mention of Ciara being transgendered.
How did the "Ciara is really a man" rumour start?
The rumour began shortly after the film clip for her track "Oh
"(feat. Ludacris) was released. Comments on the internet began to surface about Ciara's muscular physique, her toned arms, and comparitively smaller frame, which eventuated into the "Ciara is really a man" rumour.