If you take a real well-played snooker ball, it has plenty of tiny but visible scratches, say, 0.25 mm deep.the ball's diameter is 52.5 mm, so the scratches depth is about 1/200 of its diameter. If we take Earth's diameter as 12750 km, scratches like that would be features about 63 km tall, nearly an order of magnitude larger than tallest mountains on Earth. If course the mantle is smaller diameter, but unless we cannot detect roughly 50 km-tall ridges (I don't know much about seismic wave registration), it's about as smooth as a billiard ball %)