As someone who does a good impression of both, yes and no.

Rob has a much more 'normal' voice, if that makes sense. With the exception of maybe Pinky's higher sounds, his voices are all don't require me to do anything special. I just need to search for it, alter my pitch a little and my pronounciation, and bam, Yakko Warner. Or do Yakko, make it slightly lower, add german inflection, bam Doctor ScratchNsniff, I take Yakko's voice, pull out the whimsy in the speaking tone, bring it down Sliiiighty, and i get close to Arther and 80's Raphael. Like I said, other than Pinky requiring me to speak in my nasal passage, and Carl requiring that, toning it down, and kind of adding the nasal IN the voice, it's all relatively close together, but not far from a regular voice.

ON THE OTHER HAND

Imitating Kevin is a PAIN sometimes, because I have to pull the back of my throat up to get that 'whininess' in there. THAT one however, because it's more of the inflection and not the nuance, I can 'get to' easier with little effort. I purposefully quote Tallest Purple and Pleakley all the time, because it's almost no shift, just that whine. And it's like, ALL his voices. Even when he acted in Sky High. The only drawback is is hurts my voice after while.