I guess mostly I'm surprised that nearly 10 years after 9/11, some people continue to have an almost visceral response to Muslim passengers on planes. We have to remember that the 9/11 attackers did not dress in "Muslim garb" - they were wearing dress shirts and coats and jeans. They didn't stand out. So it's probably a mistake to take any clues from what people look like.
If you're on a plane, there are other things that can and probably should make you nervous - if you absolutely have to obsess. Did the pilots take a drink? Did they get enough sleep? Did the mechanics service the plane properly? What's in the cargo hold and was it packed correctly? Did my bag make it onto the plane? Is the flight on time? I mean, enough things to keep your mind off what people are wearing.