Not necessarily. A lens hood has to block stray light without intruding into the image area. Because camera sensors and film are rectangular, the lens’s field of view is effectively wider along the frame’s diagonal than along the top, bottom, or sides.

A round hood must be short enough to avoid vignetting at the frame corners, which leaves extra unused space along the top, bottom, and sides. A petal-shaped hood takes advantage of that shape: it is cut shorter where the frame corners need clearance and longer where there is more room. That lets it block more stray light than a simple round hood of the same safe size.

Round hoods are still common, especially on lenses where the shape of the field of view or the hood design makes a simple circular hood practical. In general, petal hoods are used because they can be more efficient for rectangular image formats.