I will say that Muppet babies, as well as the Preschool "WOW you're a _________" playtime videos were pretty much proof that Jim wanted to market the Muppets to kids, while he wanted to focus on creature shop type projects for older audiences. At least that's the impression I was getting.
I still have to say, some of the voices worked beautifully (Frank Welker and Greg Berg were spot on), while some fell short (Baby Piggy, I agree with you was too shrill. But she sounded like how she should after a while). Personally, a bigger contrast would be something like Fraggle Rock. The designs of certain characters, Gorgs and Majory being the main examples, were pretty off. it's hard to simulate that kind of style in limited TV animation. I think Junior and Ma looked the worst. And the voices tried to impersonate the characters. Tried. But while there were some close ones- Red and Boober, close enough ones- Gobo, Matt, Wembly, and how was that supposed to sound like the real character- Junior.
That's the big question. What if they actually tried to make MB a puppet show? it would be slightly different. Stronger at some points, weaker at others.
And on the subject of 1980's outsorced Japanese animation...(and cartoon versions of Puppets), I was watching the DIC/Saban produced ALF and I felt the animation and overall design of that show was excellent. There were even small subtile things in the back of the show that stood out. I almost wonder what would have happened if MB and FR had that same animation style.
That said, I always ask this question and bring up 2 examples. What's more important, the story or the animation? When both are going for you, you get a great project no matter what. But sometimes, one has a higher quality than the other. Pocahauntus had supurb animation. It looked beautiful, but watching it was dull. It had a poor storyline and a bad script (and a million buzzillion historical inaccuracies). And look at Rocky and Bullwinkle. Hideous animation, but it was so well written and delivered, it is still considered great. Even as an animator, myself, it's more important to have a strong story than just strong animation.