It's not that special a 'concept'- it's actually quite conventional, comparing one thing with another to produce imagery. "Pretty as a flower", "ugly as a mud fence", "fast as lightning", etcetera.
The only thing interesting about it is the writers' need to keep from using a phrase so common it's basically a cliche. In this case, phrases like 'having the fashion sense of a blind golfer' are basically cliches, plus I was kind of limited in that I didn't want to use an animal reference- 'fashion sense of a blind cave bat', maybe, or 'fashion sense of an epileptic koala'.
So, since Doc is a mechanical type, he makes a comparison to generally unattractive pieces of machinery. A "retired" dump truck is generally one that has been worn completely out- bent, dented, rusted through and held together with baling wire.