Others have pointed out that microfiber would already be considered microplastic, but maybe you’re aware of that and are thinking that microplastics are plastics that are manufactured to be microscopic (like fiberglass is manufactured as microscopic glass).
But all (petroleum-based) plastics break down into microplastics, whatever scale they’re originally manufactured at. Microfiber is no different from other synthetic cloth in that respect—while it should be avoided like other synthetics, there’s no reason to single it out.