Not always. It depends on the elasticity of the skin, the anatomy of the body and other extenuating factors. And yes, the skill and technique of the doctor plays a role. A qualified doctor can give you an idea during a consultation exactly what risk you may have of irregularities in contour after surgery.
One thing to keep in mind though is that there are two types of lumping that can occur. One type is temporary and occurs because of an uneven resolution of swelling in the tissue. Sometimes this lumpiness can be seen, others it can only be felt. It usually goes away on its own given time.
The second type of lumping happens less often but is much more serious. It occurs because of under or over resection of fat, and will require that additional surgery be done in order to even out the results. Another thing to keep in mind is that people who smoke or are otherwise not in great health can also end up having lumpy results.