I have type 2 diabetes and a high serum creatinine level. How bad is this?

High creatinine. You need to see a nephrologist (n) for an evaluation which includes a 24 hour urine collection to measure your kidney function and to if protein is in your urine, a renal ultrasound and blood work. Depending on the results, the n can evaluate your kidney disease. You also need to control your glucose levels as well as making sure your blood pressure is as low as possible without feeling dizzy.
See below. A high serum creatinine and a calculation of GFR which will be in turn low, helps to evaluate how well your kidneys are working to clear waste from your body, even if you are making urine normally. The range of GFR may be 90 [normal] and below. Below 10-15 means you may require dialysis or transplant.