I expect the operation was carried out through a Craniectomy. This term refers to the permanent removal of bone overlying the affected part of the cerebellum or hindbrain.
If this was so, the bone may never regrow completely and a small gap may remain in the bone life long. This does not really matter as there are very thick overlying muscles, which are brought together by sutures over the defect in the bone and they are more than adequate protection for the brain.
A pseudomeningocele is a collection of the water-like fluid that normal surrounds the brain in the space created during surgery outside the normal covering of the brain (called the dura). In most cases this fluid remains static or diminished over time and may even disappear.
If there is no bulge of the overlying muscles under the skin, nothing further may be necessary as regards this fluid collection.