There are many different types and causes of liver disease, and the degree of fatty liver does not fully reflect the prognosis of the disease. A mild degree of fatty liver does not mean that liver fibrosis or cirrhosis will not develop, and a severe degree of fatty liver does not necessarily mean that liver fibrosis or cirrhosis is already present and the prognosis is poor.
All other things being equal, severe fatty liver does have a higher risk of poor prognosis, but the prognosis is determined by whether the cause of the fatty liver can be removed (e.g. chronic hepatitis is difficult to remove, alcohol consumption is relatively easy to stop), whether steatohepatitis has developed on top of the fatty liver, whether liver fibrosis or even cirrhosis has developed, and whether treatment is timely and appropriate.
Even if liver fibrosis has developed, it is still possible to reverse the fibrosis with aggressive treatment.