What is the meaning of [Ask a silly question and you’ll get a silly answer]

Literal meaning.

This sounds like a fairly recently coined proverb and, in it’s precise wording, it is. Nevertheless, a medieval version with almost exactly the same sentiment written in Middle English pre-dates the modern version by a good 500 years. William Caxton expressed the notion in his retelling of Aesop’s Fables, 1484:

For to a folysshe demaunde behoueth a folysshe ansuere.

The modern translation had to wait until the 20th century, as in this example from the Minnesota newspaper The Brainerd Daily Dispatch, Tuesday, April, 1945:

If you want to ask a silly question and get a silly answer, just ask Mona Roth whom she refers to as El Dorado.