Complex sentence: They first appeared in a one-time Shelly Duvall special called "It's Popple Time", in which they were puppets and marionettes; after being well-received, it was decided to make a cartoon series with the same characters.
Split into simpler sentences: They first appeared in a one-time Shelly Duvall special called "It's Popple Time", in which they were puppets and marionettes. The special went over very well, so it was decided to make a cartoon series with the same characters.