Link is a male character, so having noticeable boobs wouldn’t make a whole lot of sense. But he is very slender and somewhat effeminate, so it’s a popular cosplay for women because more girls have a Link-like figure than guys.

The “30 second rule” is a commonly cited rule of thumb for determining fair use, but using 30 seconds or less of a work doesn’t necessarily mean that it’s fair use. In this case, it’s definitely a copyright violation; he just doesn’t seem to care.