Supposedly microwave popcorn is extra bad because of the chemicals it contains. I think an air popper that uses no oil would be ok, so it’s just the kernels in the machine.
As for salt, 75% of the salt in our diet comes from processed foods, therefore, things like cheese, frozen foods, canned foods, condiments, restaurant foods, snack foods like potato chips, etc. are the majority of the salt in our diet. Table salt, a sprinkle on a bowl of popcorn, is minimal and not an issue.