A Kaiju is just a giant fantasy monster, usually entirely from Japanese media / stories! So I don't see why an Alien couldn't be a Kaiju so long as there is some lore? Hope this helps!
Kaiju is japanese for strange beast so I would assume that an alien could be classified as a "strange beast". Not only that but in Pacific Rim the Kaiju are aliens so I am 99% sure aliens would count.
