When I built the head that became Esmerelda for TVTV, I tried to make a "pretty" mouth because the puppet was originally for something else. I failed.
After messing about trying to make a hard palette to move that slit in the foam and then covering it with fleece, the resulting mass of foam and fleece being pushed around when the mouth was opened just morphed the whole lower head and looked like a toothless old granny. It was awful. So I abandoned it until I needed a puppet head in a hurry for Esmerelda on TVTV.
Upon reflection, I wasn't letting the materials do what they do best. I was making them do something else.
If we're examining mechanics, it looks to me that the Bunsen mouth in the pic above is an open oval. That's smart because it doesn't misshape the head when you want it open. And when you want it closed, the foam stretches instead of buckling like it did in my puppet.
If you can get past the "love doll" look of the original sculpture (and not laugh the entire time you're making the puppet!), its actually a very smart technique.
I'd tend to think that if you're going with a fleece covering, you'd want to be careful about how you applied the fleece and made sure the stretch was helping you instead of impeding you when you puppeteer.
-G