Ha, the flaw in their thinking was the insinuation that anyone could be "bored" of dragons. Cartoony is fine, but I think they took it too far into the realm of goofy with many of their designs. Toothless is the only dragon I like from that movie.
Yeah, the other dragons just looked stupid. The Night Fury had that perfect mix of threatening and cutesy, in my opinion. He could look cute when he wanted to, but also look completely intimidating when he wanted to.