Is it normal to have ct scan say stable increased lobulated density in anterior superior mediastinum may represent residual disease vs thymic tissue?

Mediastinum: Of course it depends on many factors. It's normal to see thymic tissue until early 20s is seen in fewer than 50% of people over 40. This density can be possibly be anything including normal, reaction to infection, thymic cysts, or hyperplasia from systemic disease. Fortunately it is stable, but a conversation with your doc and radiologist would be the best thing moving forward.