The slopes of most cinder cones are usually about 30 degrees because

The slopes of most cinder cones are usually about 30 degrees because that is the angle of repose, which is the slope at which the loose cinder can stand in equilibrium.
The cinder cones vary significantly in height, from just a few meters of height to few hundred meters.
They are numerous around any volcano, and lava can come out of them or may flow below them.