Here’s another “not too bright” snowclone. Y and Z here correspond pretty closely to the X and Y of “a few X short of a Y”:
Not the brightest light in the harbor
Not the sharpest knife in the drawer
Not the brightest crayon in the box
Not the quickest bunny in the forest
Not the quickest horse in the stable
“Not the sharpest knife in the drawer” alone seems to be fairly popular: it gets ~37,500 Google hits at time of writing. Like “a few X short”, this snowclone doesn’t seem to have an original referent, which makes it also a “strong” snowclone. It’s a cliche, an [American?] idiomatic expression, rather than an allusion to a phrase from a specific source.
This one is less flexible than “a few X short”, where the adjectival X is intended to correspond to mental agility, playing on the physical and mental senses of sharpness, brightness, and quickness.
[from Arnold Zwicky’s list at Unblogged Snowclones]
The Snowclones Database 
(I couldn’t find a contact email, so I posting this here)
When leaving out diacritical marks, even in oft-used foreign words, it can make it difficult to know the word and/or its proper pronunciation. Cliché is actually easy to spell correctly in HTML: cliché.
You may also find the following reference helpful:
http://www.digitalmediaminute.com/reference/entity/index.php
Just FYI.
Not American idiom only, writes this Englishman.
This exists in Danish, too. A provincial version is “ikke den hurtigste knallert på havnen”, which more or less equates to “not the fastest moped on the harbor”.
i found a hilarious misquote somewhere that i was reminded of when i read this:
not the sharpest sandwich in the picnic.
i know, ridiculous.
btw, this site is awesome.