Sure, why not...who cares about internal consistancy. Hey if the reptilian decendants of a reptilian race that lays eggs and doesn't nurse its young can have a nice full set of mammaries...why not cover the darn things in hair?
Hey, this is D&D so we have to pander to cliches and equate fantasy with absurdity. :\
To add: Dragons are not platapuses and dragonborn are not decended from these creatures. They are decended from dragons, who do not possess hair and do not nurse young and in fact do not possess the secondary sexual characteristics of mammals.