Is a right superior mediastinal fullness bad? what does it mean?

Depends: If this is something reported on a chest x ray, it needs to be followed by a ct scan of the chest. Most often it is just a projection artifact (ie nothing significant), but it can be due to an aortic aneurysm, an enlarged thyroid, or a tumor.

Mediastinum: This is an area in your thorax (chest) defined laterally by the lungs on each side and forward and back by the sternum (chest bone) and the forward part of the spine, so it is a rectangular box. There are many things inside it. In the superior area fullness may be your thymus gland, lymph nodes, aneurysms from blood vessel and so on. Whether this is serious will depend on further studies. Best.