I am not buying this argument. The common people are only fascinated, terrified, see it as a devil or being from their folk tales if the DM decides they do that.
Apon laying eyes on the first Dragonborn, the common tavern owner could just as easily look up yawn and ask if he wants a private room or to stay in the common room.
I think it just depends. Let's say for a moment that aliens start visiting earth. At first, we're likely pretty cautious around them, but after a while we accept them. Meanwhile some of the aliens are okay. Vulcans are a bit stodgy but okay. Betazoids are a bit annoying when they sense emotions.
But then there are the dumb crossover aliens. Xenomorphs from Aliens that use people as hosts. Predators that come to earth to hunt people just for fun. Heck, look at the reaction to seeing Borg in Star Trek, or in the most recent series the Gorn. Look at the aliens from a dozen other movies where the aliens are homicidal maniacs who are out to eliminate humans for one reason or another.
Then an alien you've never seen that looks and awful lot like a xenomorph shows up. Do you really think people would accept them with open arms? Reactions to unknown creatures are going to vary from campaign to campaign. In a world where Mos Eisley's cantina was commonplace, maybe it wouldn't be a big deal. In a world where only a few known "safe" races exist and many monsters are intelligent and may look humanoid even if the book classifies them as fiends, I think the reaction is going to be different.
It's up to the DM to decide what the reaction is going to be based on established lore.