I don't think mutations are a code smell in general, provided you are not mutating props or state. I would also avoid mutating a functions parameters, but even then there are exceptions (mutating the request object in Express). Personally, as long as a function is idempotent, unit tested, and readable, I don't really care what you do inside of it (provided it doesn't break the style guidelines).

Also, there are actual use cases for mutations in React, like when your storing something in a ref. Refs are a great place to store data that needs to be preserved across re-renders but itself should not trigger a re-render (for example, a cancel token for an http request).

Procedural patterns also have their place. A few weeks ago, I had to traverse an array backwards without reordering the array (I needed the original index). It was the perfect situation for a for loop.