The Kurdish proverb every ripe and delicious cantaloupe (muskmelon) is eaten by a donkey is one of the most interesting proverbs that I have heard in four languages, including Kurdish, Persian, English and German. I personally use this proverb frequently.
It is obvious that the proverb is a humorous one. It is used in different contexts. Generally it is used when something good is given to an undeserving case. But, in modern usage, many people use it when they want to say that a beautiful, attractive and comely woman marries a ugly, unattractive and grotesque man. This proverb is often used by male sex. The speaker is usually someone who carries a torch for the girl. Sometimes the speaker says this proverb jealously.
A: I can not believe that Sara has married this plain man.
B: It is not strange. Every sweet cantaloupe is eaten by a donkey. She has probably married money.
Also there is a Persian proverb that resembles the Kurdish one: The ripe grapes are given to jackals.
Is there a proverb that would express the same thing in English? Unfortunately, I have not been able to find any equivalent in German either.