For your application connect the PGOOD output to a pullup resistor to your 3.3V supply and tie the signal also to the input of your MCU.
If you do not have an available 3.3V connection for the PGOOD pullup you can indeed use a combination pullup to the TPS56221 BP supply and a pulldown to GND to limit the upper voltage level of the PGOOD signal. Use the formulas for a voltage divider (considering that the PGOOD signal is not pulling to GND) to compute the appropriate resistor values. Make sure you do not use too low of resistor values so that you do not put excessive load on the BP supply pin.
An open drain output from the TPS56221 means that the device can only pull the PGOOD signal to ground (which it will do whenever the output voltage is not within an acceptable range). It relies upon an external pullup resistor to some voltage level to produce the high level of the signal when the output voltage comes into range. The reason that open drain signals are used in this instance is specifically so that the PGOOD can be adapted to the logic levels of whatever control/monitoring system is in use whether that be powered from 1.8V, 2.5V, 3.3V or even 5V.