It'd certainly be much easier to essentially texture swap Earth into Mars and the Xenonauts into the Alien Defence Force or whatever, but I think it'd miss the fundamental opportunity that comes doing an alien campaign - which is that you're actually getting to invade Earth while playing a massively superior species. Finding yourself hugely outnumbered by the primitive locals and having to beat them before they manage to steal and reverse-engineer enough of your stuff to defeat you is a pretty neat idea, particularly if you can come up with plausible reasons why the aliens choose to attack Earth in a piecemeal way that actually gives the Earthlings a chance of victory...
But whether it would actually be fun to play as a game, I don't know - would it be challenging to fight massive hordes of humans in every mission, or would it just be boring? Conceptually it's a nice idea but ultimately I don't know whether it would be enjoyable in practice.