No, people throw around titles like that too often.
People miss the whole point on Couture. Yes, Couture is. Because he never had a prime. Name one guy who has never seen his prime in MMA yet has had that much success. Not to mention, in addition to spending half his career in his 40s, he was always the MUCH smaller man at HW. So he was an old ass man who was undersized.
The fact that no one will ever be as good as COuture was in his 40s makes him legendary. Although Hendo is doing very well...he has a ways to go to reach his mid 40s and still be doing it at a high level.
Couture is legendary for it. That is the definition of a legend. Someone who defies the odds...and did something where years from now fans will be like "remember old man Randy winning and competing at a high level at age 45? That shit was crazy".
To rehash, no Machida isn't. Certainly will go down as one of the more interesting styles in MMA history. But legendary? I don't see it.