There are more complete titles of this work that give more background information. For example, Zehn Variationen über die Ariette “Unser dummer Pöbel meint” aus dem Singspiel “Die Pilgrime von Mekka” von Christoph Willibald Gluck from the Wiener Urtext edition. The lengthier title informs us that “Unser dummer Pöbel meint,” is taken from an arietta in Gluck’s Singspiel, “The Pilgrim from Mecca.” A Singspiel is a light German opera where dialogue was found throughout.
Having trouble finding the original aria? This “Singspiel” is actually a French opera comique. Mozart knew the German title and text. The aria in its original language is “Les hommes pieusement.” That should make your searching a bit easier. This gives color to the translation of the German. The fact that the French is clearly translated as “Pious Men,” shows that the German dummer becomes a kind of false cognate if translated as “dumb” or “stupid.” It is more accurately translated literally as “Our simple people think” or more precisely as “What our simple people believe.”