[quote=@thedman]
In my opinion, we should have Equestria Girls be non-canon for this, as I feel it would be less impactful for the ponies if they had already seen humans before. It would go from "oh crap, space aliens", to "oh look, its those beings from that mirror world".
[/quote]

Does it really need to be non-canon for the contact to be impactful, though? Humans or not, they still came from friggin space in spaceships after all. I would think that would be enough. Even so, it's also entirely possible that Twilight isn't going around telling everyone about the mirror world. Sure the main cast obviously knows, but Average Joe Earth Pony in Manehatten or Average Jane Do Pegasus in Cloudsdale probably haven't heard about any of the Canterlot High adventures so... blarg.