I think most would agree that multicore capability is desirable, but it's clearly not a "must" as ocaml has been without it all of this time and many have used it to good effect regardless (of course it might be a must to certain people who need it for their use case). Also, the same restriction that prevents multicore in ocaml (the GIL) also affects other languages which are obviously useful, such as Python. So to answer your question it's clearly "ok" since it doesn't prevent people from productively writing effective and useful software in it, although it is of course a restriction which many would like to see go away.

To be fair, the current multiprocess approach that OCaml concurrency libraries use handles the most common concurrency need (concurrent io) just fine. Multicore is most useful for CPU-bound computations.