The doctor is not a great man at all. He does hold some authority, but he is not particularly good or wise; nor is he a success. First we get a description of him from the point of view of the town's beggars, who always keep an eye on everything that's going on. They know all about the doctor and appear to regard him with no small measure of contempt.
They knew his ignorance, his cruelty, his avarice, his appetites, his sins. They knew his clumsy operations and the little brown pennies he gave sparingly for alms. They had seen his corpses go into the church. (chapter 1)
The reference to the doctor's 'corpses' underlines the extent of his failures as a medical man.
A little later we see the doctor at home, fat to the point of grotesqueness, and self-indulgent; hardly an attractive figure. He makes a great show of living in luxury but in actual fact he has come down in the world, as Steinbeck ironically remarks.
The doctor had once for a short time been a part of the great world and his whole subsequent life was memory and longing for France. 'That', he said, 'was civilized living' - by which he meant that on a small income he had been able to enjoy some luxury and eat in restaurants. (chapter 1)
The doctor frets at no longer being part of 'civilized living', but even when in France he was not earning much money as he had only 'a small income' there. It seems he has never had much money or success anywhere.
He is successful, he is rich and has servants. But he is not a great person overall. He poisoned Koyotito making him barf to seem like he is treating him. He also wouldn't help the baby out even when he is poisoned by a scorpion in the first or second chapter.