The definition of tin foil hats you have is fine but I think the more literal meaning is more appropriate here... essentially, people who wear tin foil hats generally believe that aliens (or "the government") are messing with our brains:
A tin foil hat is a hat made from one or more sheets of aluminium foil, or a piece of conventional headgear lined with foil, worn in the belief it shields the brain from threats such as electromagnetic fields, mind control, and mind reading.[...]
The use of foil headgear is often based on a belief that such hats prevent mind control by governments, spies, or paranormal beings that employ ESP or the microwave auditory effect.[...]
So, people wearing tin foil hats are expecting mental attacks by aliens/Martians.
Someone wearing an actual helmet would be expecting a physical attack from Mars.
The implication is that, for some reason, now that we "know" Martians exist, they will attack us.
But, the sentence after this one in the article explains why the trade of hats is likely unnecessary:
One NASA scientist says there's a perfectly good explanation for these sightings.
It then goes on to describe scientific explanations for all of the "alien" objects.
Vasavada is alluding to a phenomenon called pareidolia, which is when your brain recognizes a familiar object or pattern even though it's not actually there. It explains why people see the Man in the Moon, Jesus in their lunch, or even a tiny woman wandering around Mars.