They’d be no more distrusted than foreign humans, unless the races of your world are more isolationist than the modern dnd norm. You are free to run your game however works for your table, obviously, but it also seems like your players aren’t exactly stoked about how you want to run it.
I disagree. Look at the origins of Dragonborn, Tieflings, and Haf-Orcs. All three have origins that are from creatures viewed as monsters or marauders, creatures where parents tell their children stories to scare them, etc. Even good dragons are viewed with fear as well as awe because commoners know they are basically powerless against them. So, while cultures might be cautious of strangers, they won't often shutter their windows and lock their doors at their approach.
And my players have no problem with it at all. When I explained the reaction they would likely receive as a group, one suggested they are a fantasy version of the Suicide Squad.
So far, we've embraced the concept and, most importantly, everyone is having fun. Anyway, I am not sure where your "modern dnd norm" comes from...
Either way, the bog standard dnd world sees Dragonborn as a noble race that is more a potential ally against the dangers of the world than a threat, and while teiflings are viewed with suspicion, history has enough good teiflings that only the crappy people genuinely assume evil when they see one.
"[Dragonborn]... a world that greets them with
fearful incomprehension." (PHB p.32), so I don't see the standard dnd world seeing Dragonborn from your view. A creature that can breath fire or lightning, capable of killing "commoners", is not one quickly to be embraced by strangers.
And I have no idea what history you are referencing to counter the first line of the Tiefling description in the PHB. In my experience, I have only read about two Tieflings (from the Drizz't books) so far: one was a dominating evil warlord and the other a vile warlock who managed to change, albeit still not really "good".