I don't think this really counts as a spoiler, but if you haven't seen Equestria Girls yet, and still plan to, you might want to consider this as a warning.

So, in the movie, Pinkie implies that Twilight has a counterpart in the human universe.

If she managed to get to the mirror, and somehow (for whatever reason) crossed into the pony universe...

Would she be an alicorn, or a unicorn?

1690518 I choose to believe that the fake one would evaporate in a horribly painful way.

I found EqG to be mediocre at best

1690527
As did I.

Since magic doesn't seem to exist in the human world, you might want to consider that she might become an earth pony. On the other hand, since Twilight presumably resembled human twilight...

Probably a unicorn. Becoming an Alicorn was something that wasn't intrinsically part of her, just something she became.

A better question is what would happen to Spike? Would he become a baby Dragon but retain the intelligence of a dog?

1690518 I say unicorn, since that's Twilight's natural state, while her Alicornhood was some variety of magical achievement on her part.

1690518
Since this inversion of the scenario was never meant to happen, I don't think there is enough information to discern what would happen.