Well, the terrible animation combined with the fact it only had 13 episodes, which is significantly less than the other two series, my guess is that they had a much lower budget than the previous shows and as such, the animation department suffered under the lower production costs.
Super Mario Boards
Phantom of the BWAH-Pera
Sally Acorn (Pre-Genesis)
@Barby Koala @twistrawberry Well, actually, I looked it up, and Adventures of Super Mario Bros. 3 technically had about the same number of episodes as Super Mario World while Super Mario Bros. Super Show had many more episodes than both series (Mario Bros. 3 and Super Mario World), but yeah I agree the quality with Super Mario World isn't as good as the two previous shows.
Aniki
Even as a kid I could tell that the Mario World series was of lesser quality than the others. I'd hate to see how it looks now with adult eyes.