For MP3423, we only have Theta JA and Theta JC listed in the datasheet. It is measured on JESD51-7 standard.
For Vin=3.4V, Vout=5V, and Iout=2A, the efficiency is around 95.5%. The power loss is about 0.45W. The Tj is calculated to be 36C above ambient temperature based on the JESD standard.