Post pregnancy hyperpigmentation or melanosis mainly due to nutrtion secondarily due decreased Hb%, so check s.
ferritin level, s.vitB12 level,s.vitD level &
complete blood count.
Derma dew glow cream alone wont help you need to use suncreen also in the morning...
Regarding that white spots usually very rarely seen by using this cream...so better you use the suncreen in morning &night this cream.
Next Steps
consult a dermatologist once & than use the cream...along with that check the above mentioned tests if its level decresaed means you need to take supplements to improve.