Question: Is the writer employing a mixed metaphor here?

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> A part of her was sinking languidly down into the passive pleasure of having returned to the familiar—**like a pebble**, she had been picked up and hurled back into the pond, and sunk down through the layer of green scum, through the secret cool depths to the soft layer of mud at the bottom, sending up bubbles of relief and joy. A part of her twitched, stirred **like a fin in resentment**: why was the pond so muddy and stagnant? Why had nothing changed? She had changed—why did it not keep up with her?
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Answer: 
I would describe it as a failed attempt at an [extended metaphor](https://literarydevices.net/extended-metaphor/).

It is certainly mixed in the sense that pebbles don't have fins.
There are many many figures of speech.

But "like" is a "simile" and **simile** describes something as being **similar** to something.

It directly gives information to the readers to let them know how something can be similar to something.

For mixed metaphor, you can access here : <https://www.grammarly.com/blog/metaphor/>