So you would possibly have (albeit rare) a tiefling in oerth if they were explicitly descended from graz'zt? He has some of the earliest cannon entries for signifficantly populous tiefling lines (even though this might seriously rankle 5e players as tieflings are an exclusively infernally touched race now whereas in the grazzts rowdy college days he was causing them to exist left and right)
I would, but not as a PC. In fact I allow tieflings as a PC race in my current GHK campaign, but only when they descend from some Batezuu, so have devlish origins. Those who are demonic offspring are the enemy, since major plot arc is party vs Iuz schemes.
If there are many demons running around, like in Greyhawk in those areas where Iuz rules it totally makes sense that some may have offspring. In fact Iuz himself is some kind of tiefling, since his mother at least once was human.
Devils might be interested what the demons are doing on Oerth and have their own agents in play and create offspring as well.
So in this case this absolutely makes sense, but dragonborn? Where did they hide all the time? Did some spelljammer crash which brought them? Nope I just ignore them, they might be fun and make sense in different settigns but not in Greyhawk (unless from a crashed the spelljammer, but never native).