Jerry Nelson stop doing his "Muppet Show" Muppets quite some time ago, he didn't do Floyd (I believe it was John Kenndy, and I think he did great), not to mention he also didn't do Lew Zealand as that was Bill.
Although that Robin voice did sound somewhat odd, I think it's awesome they even put him in there. Maybe there going a new way with Robin, maybe there going to make it so he's not five anymore, makes sense to me. Maybe their making out to be a pre-teen or maybe around eight or nine years old. I think it could work