Based on how there don't seem to be any reports on Alolan Rattata-disguised Ditto, I'm inclined to say that it's not a possibility. I don't think anyone really has more information than that (unless someone has witnessed one) so I can't say for certain, but based on the lack of information about it and the fact that it'd be unique from all the other possible transformations I'd say it's just regular Rattata
I don't know if you're okay with that as an answer or if you'd like to see if anyone else has more conclusive information.