I think there's unanimous agreement that by the mid-1950s, there's a noticeable decline in quality. The early 1950s weren't as anarchic as the 1940s shorts that preceded them due to staff changes, but most of the cartoons are still solid. Some of my favorite Lantz cartoons are from the 1950s - "Convict Concerto", "Termites from Mars", "Crazy Mixed-Up Pup", the early Chilly Willy cartoons...
Paul J. Smith is by far the weakest director of this decade, but I'm willing to defend some of his earlier efforts, like "Arts and Flowers" and "Niagara Fools". His later works tend not to stand out in any way, and fall flat as a result.