>>104941580I'd say humanity at large accepts that aliens exist, they've popped up frequently enough. Sizz-Lorr blatantly announced he was going back into outer space. If you go by the comics they've appeared a couple times, and Zim has been seen without a disguise fairly frequently; he was seen by an entire auditorium full of normal, not-crazy citizens once. People in the Zimverse are also dumb, so they probably assume anybody claiming to see an alien is most likely lying for attention, since they aren't THAT common.
