Here's why:
* Metaphor: A metaphor is a figure of speech that compares two unlike things without using words like "like" or "as."
* Simile: A simile uses "like" or "as" to make a comparison.
Since the phrase doesn't directly use "like" or "as" to compare something to a bug in a rug, it's a metaphor. It implies that something is extremely cute, just as a small bug might appear cute and harmless when nestled in a rug.