Yes.
A lot of people were speculating on this before, and it was confirmed a while ago when we got a glimpse at Diancie's Pokedex entry:
As you can see, it states that it is a sudden transformation of Carbink, meaning it was a Carbink before. This doesn't mean Carbink evolves in to Diancie Though, Diancie is probably just a special occurence among Carbink.
Hope I helped!