Moon knew enough of Earth that she felt it a safe place to let Star live with the people there,
Tom is intimately familiar with human culture including their music and movies, as well as being able to travel freely between Hell, Mewni, and Earth, and pretty much anywhere else other than the realm of magic
and if demons are both real and spending some amount of time on Earth then surely over the past 300,000 years at least some occultists would have sought them out, learned actual things from them, and written it down. Tom doesn't even bother to hide his horns, third eye, or abstain from using his powers in public, and nobody cares, so there's not even a masquerade of any kind.
If the show depicts demons and magic as only slightly surprising to the average citizen all you can conclude is the average citizen is fully aware that supernatural things are real with about the same certainty that they are aware Jupiter has moons