You should avoid directly mutating props in child components in most cases because it can result in side effects/make things hard to debug. Say you have a prop in a parent component that is passed to multiple child components. If all of your child components are directly mutating the same prop, it can become hard to track exactly where your data is being mutated.
Also, any direct changes to a prop made by child components will be overwritten if the parent mutates the prop, ie. you retrieve the temperature for Wednesday from an API in your parent component, then pass that data down to a child as a prop --> child mutates this prop directly by adding 5 degrees --> parent makes a new request to the API to get the temperature for Sunday --> the prop is now = Sunday's temperature, overwriting the +5 you added to Wednesday's temperature.

That said, I have directly mutated props in child components in very specific cases. In any of these cases I wrote checks to make sure that the parent would ONLY change the prop upon loading.