Why should we use Kotlin coroutines when we have mutiny-kotlin ?

Good question, because Mutiny is a library, and Kotlin Couroutines a language specific way of running async code. Both will lead you somewhat to similiar results from my point of view i would always prefer coroutines, since they are way better to read, and i dislike the verbosity of mutiny.

Because of KMM or KMP or KN in general

this is the first time I hear from mutiny , does it support structured concurrency? otherwise that'd be a reason in favour of coroutines as well.

Mutiny is really a stripped down RxJava/Reactor. It's more of a streams based way of thinking rather than a coroutine way of thinking of async work.