Similar to the introduction of a short, feisty character named Scrappy Doo on The Scooby Doo Show; once Mickey was introduced to Seinfeld, the show possibly also started it's slippery slide...
Mickey's friend, Kramer was lanky; as was Shaggy.
http://www.test.centrinity.com/~mark_carey/stan/mickey.jpg
http://www.angelfire.com/ab2/almightyfrog/images/scoobscrappy.jpg
In "The Stand-In", where Mickey and Kramer do actor stand-ins, there is also an appearance by a little boy with a "bowl-style" hair cut; as if his hair was cut while he wore a bowl on his head.
Isn't the introduction of a boy with a bowl-cut traditionally a death sentence for a TV show? [Maybe it wasn't exactly a bowl-style cut, but tragically, perhaps enough bangs to suck the very life out of the Seinfeld series.]
In "The Stand-In", the presence of the tough little sidekick Mickey and the bowl-cutted little boy might have proved to be too lethal of a combination for Seinfeld's longevity. ...
P.S. I don't know how that show Eight Is Enough defied the laws of physics by having an appearance from the very beginning/every episode of a boy with a bowl-cut.
They were apparently spared...