Not so sure about a flop though for the Jetsons...
Oddly enough, it was. It actually originally only lasted one season (24 episodes. I just checked) and wasn't really considered a success evidentally. But, at some point they started syndicating them and over time it just started to get popular after all, so they made a couple extra seasons for syndication in the 80's.
What exactly is
it with sci-fi shows doing badly initially and then gaining fanbases after cancellation, anyway?
But anyway.....there isn't really an overall trend with cartoon failures. Sometimes shows work and sometimes they don't, and even good creators can make a big mistake, or just not get the tone quite right, or even just have their vision be completely raped by committee/business thinking. Generally I think you'd have to go down show by show, and in doing so I doubt you'd find a serious trend, even just factoring in recent shows. "American Dad"'s bad reviews are easily explained by its sucking. "King of the Hill", though starting to die, is in its eighth season I believe, which is much farther then even most successful shows go really. Its only just now that its starting to fail, and a lot of that is (suprise) mishandling on the part of Fox. Futurama, though it struggled, was really still more successful then a lot of people give it credit for, considering it lasted five years, which is almost an eternity by network tv standards. (and as for The Simpsons, it would be hard for anything
to match the massive success and impact The Simpsons had, if one must compare the two shows. TS pretty much revolutionized tv) With some of the failures, I think it comes down to those involved misinterpereting who their audience is, or somehow not connecting with the audience in the same way, or just plain having their creation hijacked by studio thinking.