Where does the phrase ding a ling come from?

It is an onomatopoeia - that means it's a Word (or phrase) where the Words sound like the noise they're trying to describe. "Ding a ling" is the noise a bell makes, so you say that, or you say "ding dong" or "ring a ling."

If you mean the slang term meaning someone is not very bright, it comes from the old idea that crazy people heard noises, like bells ringing, that were not actually there.