Screw-on freewheels wobble because it's almost impossible to make perfectly aligned threads on both the hubs and freewheel bodies. That means that the freewheel's axis of rotation is slightly mis-aligned with that of the hub shell, and it therefore wobbles slightly when the hub is spun. It's perfectly normal, isn't fixable and doesn't affect function unless it's extreme.

Cassettes of the most common designs wobble for similar reasons of axis mis-alignment, and generally only those supported on the axle on their own bearings and independent of the hub shell are wobble free.