Well, we know that Jim helped create Yoda, and was originally offered the part, but he was too busy with his own projects to do so, and offered Frank as an alternative.
And that too brings to mind how voices start sounding the same after a while (as just previously mentioned), like Caroll has pointed out Yoda sounds similar to Grover, and we can attest there are similarities in Kermit and Ernie's voices, in Fozzie and Bert's voices, in Rowlf and Dr. Teeth's voices, etc... I've said many times before, it seems like whenever Frank plays a one-shot, random AM/Whatnot character, he almost always seems to use a Bert/Fozzie-esque voice for that character.