Thanks to Viacom's annoying robot gateways, I wasn't able to watch the video, but it piqued my curiosity, so I did a bit of investigation on both artists.
I can honestly say that this is the first time I'd ever come across the name Jason Derulo (sorry, Jason, I mean no disparagement to your talent or popularity - just that your music is some way off my radar). Judging by the clip I watched, it wouldn't surprise me at all if Carrie's "new sound" did incorporate some similar musical influences (which are coming into modern Country faster than the Wabash Cannonball) - but I think those influences would be musical, rather than vocal.
I had heard the name Colbie Caillat - but never listened to her before either. She's the daughter of the guy who co-produced two of Fleetwood Mac's classic albums. Her music contains folk influences, and is at least sometimes acoustic led. She also makes a substantial writing contribution to her albums. I listened to one of her co-writes with Kara DioGuardi. Again, respect to her, but I don't see similar work as being any progression for Carrie.
Not having seen the original clip, I'm unsure whether "copy cat" is meant to imply that they're sounding like Carrie, or that Carrie is sounding like them - but on the basis of my brief sampling, I couldn't find a significant vocal resemblance, in either tonality or style.
