Many of the artists of MAD have had this happen over time, particularly those who have been with the magazine the longest.
Mort Drucker in particular used to be a lot smoother in the 1950s before taking its somewhat more jagged (and more detailed) appearance in the 1970s.
Al Jaffee's early work was a lot less round and cartoonish until about the early 70s. Considering he's in his 90s, he's still maintained his style pretty well, although some of his 21st-century work does have a slightly muddy look.
Dave Berg's art became a lot rougher and less detailed in his later years, also due to old age taking its toll on his motor skills (he died in 2002 at age 81).
Don Martin's early work was a lot stiffer-looking and somewhat less zany. He also didn't use his trademark Written Sound Effects early on.
Duck Edwing's style changed noticeably over time — he used to be more cartoonish but toned it down slightly (for one, he doesn't draw noses quite as big as he used to). He also switched from hand lettering to computer lettering.
Bill Wray's art for the Monroe and… series became a lot more rushed late in its run, as the comic was often forced upon him with a very tight deadline. Once Mad switched to color, he even had to color four pages in less than 24 hours. The feature eventually ended, then made a brief return with another artist.