Hi,
5V, 15mA output makes 75mW.
With the voltage drop of the catch diode included it may be 5.5V x 15mA = 82.5 mW.
82.5mW at 3.3V = 25mA
Now add the current for the Opamps and the complete control loop inside the IC, losses in the switching FET and losses in the coil...
Then 31mA is a typical value.
The output ripple of 1mV suggests that you need a constantly switching instead of power saving burst mode.
Sorry, there is not much to improve.
Use a synchronous rectifying controller and safe max 2mA, allow more output ripple, select a micro power converter IC and maybe save on other mA.
Klaus