Just now found this. I'd veer more towards "Audio Voice Actors" if it wasn't matched to a specific Muppet (even though the visible crowd is Muppets, it's basically what's termed walla, background noise or sounds). But that's me.
The bigger issue is whether it's her. I'll try to give a listen at some point. Since it was the same episode she was in, it's possible, but I'm not sure how easy it would be to distinguish and determine if it was her. I'll weigh back when I can hear (if there's a clear individual "Boo" at some point that can be compared to Ross and determined not to be any other Muppet Show cast or crew member, maybe, but that's a lot harder than a single line which aren't easy to determine in themselves; for Reeve we had a source, so whether we could hear it or not mattered less.)
Or I know some of our members know how to isolate or slow down sounds, so that may or may not help. (I'm really not sure where specifically Ross's voice is supposed to be without carefully examining the whole episode).