STM32L071KZ battery voltage monitoring via ADC
You need to make sure the batteries you use will never exceed 3.3V. If that is a possibility with fresh batteries you should consider adding voltage divider to bring ADC input down to acceptable range. This will increase power consumption from the batteries though. The usual cure for this is to add MOSFET switch between the battery and ADC, controlled by another MCU pin. The MCU will open the switch periodically, measure voltage and close it back. See this question for example.

Also the diode between Boost DC-DC and Vdd will drop some voltage (depending on current and diode), so you might want to adjust boost output to slightly higher voltage.

Note that Boost converter will drain battery even if USB is connected. Furthermore, your LDO must produce higher voltage than DC-DC for diode to close, something like 3.4~3.5V

So, i would suggest either use barrel DC adapter and a jack with mechanical switch that will disconnect battery when power plugged in, or another MOSFET switch that will disconnect battery when USB voltage present. See this question for relevant discussion.

Other than that your circuit looks OK.