I was watching Dumb and Dumber, and Lloyd said "Water under the fridge" instead of "Water under the bridge" ( That's all water under the fridge now, Har. Think of the bright side. You're finally getting to meet her, and you never had to change all those poopie diapers )
I believe the expression is 'bridge,' not fridge, is it a misquote common expression for comedies to sound stupid ?
You are correct. The idiomatic phrase is "water under the bridge," which means
"Something that happened in the past and cannot now be changed."
From: http://dictionary.cambridge.org/us/dictionary/english/water-under-the-bridge
You are also correct that it is common to misquote common expressions in comedies to sound stupid for comedic effect.
You might also want to Google "malapropisms" to find other amusing examples, such as these:
"He had to use a fire distinguisher.
Dad says the monster is just a pigment of my imagination.
Isn't that an expensive pendulum round that man's neck?
Good punctuation means not to be late.
He's a wolf in cheap clothing.
Michelangelo painted the Sixteenth Chapel.
My sister has extra-century perception.
"Don't" is a contraption."
All of these are from http://www.fun-with-words.com/malapropisms.html
Well, in this case it is meant to be amusing. Mis-speaking idioms is often silly because we expect one thing, and get something other. You can think of this as a very simple example of absurdism.
To answer your question about this phrase, no; it has been done before: trailer park boys
