Is there an English equivalent for this Kurdish proverb: "every ripe and delicious cantaloupe is eaten by a donkey"

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.

Rosalind Fergusson, The Facts on File Dictionary of Proverbs (1983) lists two proverbs that seem exactly on point, although I have never heard either one spoken in the wild:

Into the mouth of the worst dog, often falls a good bone.

and

The worst hog often gets the best pear.

Since Ferusson doesn't specify where her proverbs come from, I was a bit suspicious of these two. However, G.L. Apperson, English Proverbs and Proverbial Phrases (1929), reprinted as The Wordsworth Dictionary of Proverbs (1993), cites "Into the mouth of of a bad dog often falls a good bone" in proverb collections from 1639 (Clarke), 1670 (Ray), and 1732 (Fuller); and Wolfgang Mieder, A Dictionary of American Proverbs (1992) notes a recorded occurrence of the same proverb from New York state, although Mieder say that the saying originated in France.

As for the hog-and-pear proverb, Bartlet Whiting, Early American Proverbs and Proverbial Phrases (1977) reports an instance of a similar saying, "The worst pig often gets the best pear," in an English periodical from 1809; other sources claim that the proverb originated in Spain or Italy.

In any event, the two sayings cited above have been recorded in English for at least two centuries and thus may qualify as naturalized English proverbs.

UPDATE (September 2, 2017)

With regard to the Englishness and the proverbiality of these two sayings, Wolfgang Mieder, The Prentice-Hall Encyclopedia of World Proverbs (1986)—which identifies the country or continent of origin for each of the proverbs it lists—has these entries:

3977. Into the mouth of a bad dog often falls a good bone. English

...

12492. The worst pig often gets the best pear. English

although it also has listings for "A good dog never gets a good bone. French," "The pig snatches the best apple. Yiddish," and "The worst pig gets the best acorn. Spanish."

Whether a saying that appears in relatively few published works may nevertheless qualify as a proverb is a question that invites subjective answers. But multiple proverb collections that I consulted include one or both of the sayings given in this answer as English proverbs.

Nice Guys Finish Last is often used to mean that women seem to prefer men who treat them poorly to men who treat them well, even though it originated in baseball (attributed to Leo Durocher) and can be applied to many other areas of life besides romantic partners.

In Marcia Gage's Soulmate Hunting After 40, the phrase is explained and supported:

Women do not always have the best reputation when it comes to romantic interludes either. It seems there is some merit to the myth that nice guys finish last. In studies with mostly younger women participants, it does appear many women bypass the sweet thoughtful guy for the macho jerk. The good news is most of the relationships are (sometimes intentionally) temporary.

It's not always used to describe romantic relationships. In addition to the sports environment where the maxim started, it is often used in business. Craig Hall writes in The Responsible Entrepreneur:

Throughout my childhood, I constantly heard that "nice guys finish last." The popular thinking was that to succeed, one must be tough, selfish, and ready to do whatever it takes to beat the other side. Somehow, I always sensed that was not correct.

So there is some overlap between cantaloupes are eaten by donkeys and nice guys finish last, but the Kurdish saying applies to more kinds of undesirable men and the English saying applies to more areas than just romance.

Beauty and the beast is a proverbial saying that is sometimes found applied to an ugly man with a beautiful wife, as in this picture -- see title and read this description at the bottom of the page:

Description: A humorous take on “Beauty and the Beast” - a very ugly man with a beautiful wife gets the wrong end of the stick. From “Punch Almanack for 1865”. Punch was a British magazine newspaper founded in 1841, famous for its humorous and satirical cartoons which were created by some of the foremost illustrators of the day: the Almanack was a supplement.

And this article (scroll down to middle paragraphs):

The study suggests that most ugly men who married attractive women were happy to bask in the glory of their partner’s beauty and enjoyed the prestige of having a beautiful wife. (...) Women have always been fascinated by The Beauty and The Beast syndrome.

And in this article which conveys the same meaning as the previous one:

[Why women want ugly husbands] We’ve all seen this situation: an exquisite, drop-dead gorgeous woman walking hand in hand with a dude who is much, much less attractive than she is...right? Kind of like a Beauty and the Beast-type situation, but not as ugly? I know I definitely see it on a daily basis.

Although 'beauty and the beast' is actually a sentimental fairytale, the expression seems to fit your description rather well, in a humorous and/or ironic sense:

A: I can not believe that Sara has married this plain man.

B: It is not strange. It is Beauty and the Beast.

If you want to focus on the monetary aspect, there is this proverb:

A golden key unlocks any door.