Is 23 degrees thoracic and 16 degrees lumbar scoliosis considered severe scoliosis?

No. No, this would be considered a relatively small curve. If the person still had growth left, they would still need to be followed to make sure it did not get bigger. If the person was skeletally mature, it would not even need to be followed. Thank you for your. Question.
No. That amount of curve would be mild to moderate based on your age and amount of growth left it would probably just be monitored but if there is further progression of the curve, you might need to wear a brace to prevent curve progression.