A jarring contrast between a precious, pure object and a filthy reality highlights a profound truth about human nature. External virtues completely lose their value when they are attached to a corrupt or mindless internal essence, lacking good advice, understanding, and refined character [מצודת ציון].
The primary approach among commentators is that the physical image presents a simple and powerful idea. A pig naturally wallows in garbage and mud. Placing a piece of gold jewelry on its snout does not make the animal beautiful. Instead, the pig soils and ruins the gold, while the jewelry only serves to highlight the creature's ugliness and abnormality [אבן עזרא, ביאור שטיינזלץ, מלבי״ם].
In the same way, a woman who possesses physical beauty but lacks sense, good character, or sound advice gains nothing from her appearance. External beauty is entirely incapable of covering up internal ugliness. Furthermore, a lack of understanding often leads her to use her beauty for forbidden desires, effectively dirtying her own virtue [רלב״ג, מצודת דוד]. Just as the physical sense of taste determines whether food is sweet or bitter, a person's speech and behavior reveal their internal character. When it becomes clear that her traits are poor, her outward beauty becomes completely worthless and deeply disappointing [מלבי״ם]. Taking this to a physical level, anyone drawn to such a person solely for their looks, while ignoring their flawed core, is like someone trying to kiss a piece of gold jewelry but being forced to press their lips against a pig's snout covered in filth [אלשיך].
Offering a surprising alternative perspective, one approach suggests the opposite dynamic. Just as a pig searching for garbage will reject a piece of gold because it cannot be eaten, a person consumed by lust might actually reject a beautiful and modest woman. To a corrupt mind, her virtue makes her unappealing because she lacks the drive toward sin and desire that he is looking for [עמנואל הרומי].
Moving beyond the literal meaning, commentators agree that these ideas form a deep allegory about human potential. Physical beauty represents the pure material, natural talent, and readiness that God plants within a person to receive wisdom. The pig, by contrast, represents wallowing in the physical desires of this world. A person who is given immense intellectual potential but chooses to chase animalistic urges rather than pursue wisdom corrupts and soils their God-given gifts [רלב״ג, עמנואל הרומי, אמרי דעת].
More specifically, this concept serves as a sharp critique of religious scholars whose character is corrupt. A scholar who strays from the proper path abandons the Torah and its core values [רש״י]. When an intelligent person studies the Torah but uses his wisdom to deceive others [מצודת דוד], or acts with cruelty and wickedness, his learning does not bring him glory. On the contrary, it condemns him even further. Spiritual and intellectual knowledge becomes disgraced when it is held in such an impure vessel, much like a pig wearing a ring of gold [מלבי״ם].