No, that is not the case. A horse who is a mix of many breeds is simply a grade horse.
A "mustang" is generally considered to be a feral horse who has lived in the wild. They are also a mix of a variety of breeds, as horses have been released into wild herds for several centuries to change the look and movement -- draft horses, cavalry mounts, thoroughbreds, arabians, quarter horses, just to name a few, have all contributed blood to the American mustang herds, so the phrase "purebred mustang" is a bit of an oxymoron.
Solaris -- 16 hh Appendix Quarter Horse = MY DREAM COME TRUE!We Are Flying Solo