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
Sleuth "Doggy" Dawg
Knuckles The Echidna
@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.
Itsameweegee
The SMW animated series has more errors than SMBSS and TAOSMB3.I also hate Yoshi's voice, Isn't he supposed to be an adult? and why are there caveman here, THIS ISNT THE FLINTSONES!