Yes, they could be demons, or zombies, or mutant squirrels, or a collective allucination, or deranged grannies with pepper sprays. If you go solipsist, they could not exist at all. But the nº 1 interpretation is that they are aliens, since they have all the characteristics of aliens, UFOs can be seen in the film, and nothing contadicts the alien idea. You can rationalize them not being aliens, but again, you can rationalize battlefield earth being a good movie (you´d need however some practice before, perhaps starting with less taxing films).
And again, it´s not important if they are aliens or not, since we agree the point of the film is how Mel finds God again, but if you hit the audience in the face with a nonsense of that caliber, you´re distracting them from what´s important to the accesory, in this case from how God did such a great thing screwing Mel´s family to how idiotic are the aliens invading unprotected a planet covered in a 75% with acid, in wich acid falls from the sky in variable, unpredictable intervals, with an atmosphere that contains acid vapors, and with the purpose of eating creatures composed in their majority of acid.