I knew the fat lady looked familiar so I went to Imdb. She was the fat woman on Married with Children that bothered Al in the shoe store. Also the principal on Popular. Yeah fat lady character actors!
As I recall, Tim Curry was pretty funny in the later episodes. I think they just let him ad-lib as he pleased. He made the show bearable.
The goofy dumb guy was fired from the show halfway through its run. The excuse given for his character's departure was essentially, "I MUST GO NOW, MY PLANET NEEDS ME." Not kidding. The other kids were left to deal with the revelation that he had been an alien all along, which they pondered for about three seconds before his replacement showed up.