It isn’t as dumb as wearing a mask outdoors, but it is close. Airliners have very clean air.
You can still get sick from the people sitting right around you.
For example, 24 years ago I flew from Boston to Cancun. The woman in the seat directly behind me was coughing the whole ride down there. Two days later I woke up with the exact same cough/cold virus. I spent the entire day in bed with a fever in the hotel room. A couple days later I felt better but then came down with Montezuma’s revenge. I still refer to this trip as the “Vacation From H#ll”.