It was a Dr. Seuss related show with the help of the Muppet company creating and supervising the Muppets designed to resemble all of the characters in the Seuss universe. During Season 1 contain entirely original episodes with stories not adapted from any book but feature at least one recognizable character such as The Grinch or Horton the Elephant. While Season 2 was considered a lighter approach to the series with a formula later used for "Bear in the Big Blue House" except with The Cat in the hat as the host.
Why was this series so underrated? I'm wondering the same thing as well. It was either because of the sudden change of format from non-moral story telling to educational story telling. Or it was because it aired on prime-time on Nick when it was more for Nick Jr.
Whatever case, I thought it was a unique thing that happen to Dr. Seuss's books and there will likely won't be another show like this one again.