What does the behavior of the people in Perfume by Patrick Suskind tell us about humanity as a whole ?
Fabulous question! You are right in suspecting that people's behavior can tell us something about humanity, especially in Perfume. Now, it's important to start with the full title of the work (because that is significant): Perfume: The Story of a Murderer. Once we look at the behavior of the people here, their behavior will show something quite dark about humanity: that people can easily be puppets, controlled by instinct... and something as simple as a scent.
Let's look at some specifics from the story where people are controlled by instinct. Controlled by instinct, the people brand Jean-Baptiste Grenouille because he has a lack of smell. Controlled by instinct, even Grenouille's mother (who births him in a fish market) wraps him up in order to destroy him or make him invisible. Controlled by instinct, Grenouille kills his first virgin to gain the scent and then must kill more to increase his elixir. Just look at the control Grenouille inspires during the orgy that gets him out of execution:
Grandfather with virgin, odd-jobber with lawyer's spouse, apprentice with nun, Jesuit with Freemason's wife--all topsy-turvy, just as opportunity presented.
Here we see, point blank, that ALL members of humanity, regardless of class or religiosity or gender, are easily controlled by instinct. While Grenouille feels successful in his escape attempt, he is also disgusted with this truth about humanity. It is even true about HIMSELF, as he is controlled by his addiction to scents.
We can also see "humanity as a whole" in Grenouille at the very end of the story. Grenouille covers himself with his most successful scent (made from the skin of virgins, the very same one that helped him escape death) and returns to the slums where he began. And what happens? He is absolutely devoured by the inhabitants of those slums. This is the way he is killed. Do his nice clothes matter? No. Does his talent for making fine perfume matter? No. Does his talent for fooling even the smartest of people matter? No. Grenouille himself cannot rise above the condition of humanity: people are controlled by instinct. For, controlled by instinct, the people of the slums kill Grenouille.
Further, the eventual reaction of the town (again, ruled by instinct) is horrifying!
The town had forgotten it in any event, forgotten it so totally that travelers who passed through in the days that followed and casually inquired about Grasse's infamous murderer of young maidens found not a single sane person who could give them any information.
Once the threat is gone, the people instinctively return to their normal lives. They no longer have "thoughts" about the tragedy. It makes it easier to go on living, especially by people who are easily controlled. People here are either too "busy" to deal with the past tragedy or too "shallow" to deal with the past tragedy or they "bury" the past tragedy, all in order to go on living as happily as possible.