I'm glad they dropped it. It just didn't look right on Season 5 Bart to be animated like a Tracy Ullman short. And I'm glad the series proper didn't look like TU shorts (after the first season, anyway). Bouncy animation just doesn't work with the Simpsons as a TV show.Those are called Twisters and it was just a strange stylistic choice which they dropped because it was hard to animate (I THINK that's what they said) and distracting. They talked about them on one of the dvd commentaries. (I'm a nerd and have watched all commentaries on all episodes up to season 11).
Though I am glad they occasionally bring back Bart's bottom row of teeth over his upper lip face.