Apparently, during each episode of The Pack (and during their live appearances at venues such as Madison Square Garden), the Evil Ninjas would constantly attack the Pack members, and were easily defeated using martial arts and weapons.
Aside from the younger audience, most of the public certainly saw the Evil Ninjas as hired actors. One of the actors playing an Evil Ninja was identified as Harvey, and seemed to be on friendly terms with members of the Pack. One can assume that he believed them to be only actors as well.
The Manhattan Clan, however, was new to the modern world when they began watching The Pack and felt the entire show, including the Evil Ninjas, was real and legitimate. Their experiences with the Pack made them see that they shouldn't believe everything they saw on television after they discovered the Pack were far from heroes. Many young fans of the show saw otherwise, and continued to be fans of the show long after its cancellation. On one occasion, Wolf convinced Billy and Susan Greene that the gargoyles were monsters sent by the Evil Ninjas. ("The Thrill of the Hunt")
Being antagonists for a show-within-a-show, they largely appeared to be faceless individuals with little to no character development (quite the opposite of the antagonists within the true show, Gargoyles).