Possibly the shortest poem ever written was variously attributed, including to Ogden Nash, and it's on fleas:
Adam
Had 'em.
Cheers,
John
Dorothy Parker wrote several short and pointed poems, such as
Men seldom make passes
At girls who wear glasses.
And there are also all the Burma-Shave ditties from the 1930s, which have their place in American history:
http://thewhynot100.blogspot.com/201...ave-signs.html