As a general statement, being overweight is suggested to make whatever disease or condition people have somewhat worse or more evident, etc. The more overweight they are usually the higher the risk. Being overweight is often associated with eating foods that are not on what would be considered a "healthy list"—high-calorie (and sugar/fat) foods, fast food, fried foods, etc.

These all cause inflammation, and inflammation is not helpful for any disease. With MGUS and cancers, many studies suggest or have proven that lowering inflammation, eating a low-inflammatory diet, exercising, and maintaining a normal weight will all help either prevent the disease itself or improve the symptoms and severity of symptoms or progression in many diseases.

It's similar to a person saying "Does smoking or drinking alcohol make my disease worse?" In most cases (peripheral neuropathy and cancers both) it is suggested that the patient NOT smoke or drink alcohol, to help improve their risks and/or their symptoms. Exceptions exist where a person smokes 3 packs of cigarettes for 50 years and never gets sick or has cancer, etc. They are rare. I would rather err on the side of doing what has a better chance of helping my situation rather than hurting it. It's a choice people make.