There is a hyperbole which says always a best husband/wife belongs to somebody who really doesn't deserve him/her! For instance, a quite gorgeous girl with a high educational degree and good family, reasonable job position etc. who has an absolutely average-looking husband who has no property or asset, nothing special in his life, no good job, not a justifiable university degree etc. (The same goes with a husband.) I wonder if there is any common English proverb / idiom / phrase that can encompass and explain the above fact/belief?
None that are gender specific. "She's too good for him" is possible, but then so is "He's too good for her".
There are related expressions, for example:
(cast) pearls before swine
Generally this is used when you present something valuable to someone who does not (or can not) recognize its worth, but it can be used in this situation to imply that the man does not appreciate the value of his partner.
Of course, that doesn't mean you can't write a song about this kind of situation, or create a metaphor.
She's Dom Perignon and he's Schlitz Malt Liquor. I don't know what she sees in him.
(Edit) The metaphor doesn't have to be alcohol-related, it could be anything where one side is if high quality and the other of low quality, e.g.
She's a ten-carat sapphire and he's just a shard from a broken beer bottle
She's a purebred Russian wolfhound and he's a street mutt with the mange