I mean, I can't say that they were bad characters. But they definitely didn't try to develop them that much either. Kind of just went with the story right off the bat. With how they made the anime, I thought there character development was about right. They couldn't really do character developing with Yuno, since she was mostly supposed to be a mystery until the end when you figure everything out.
They made Yukkii a little bitch, and played that part well throughout, but I can say for sure they could have done his a little better than they did.
And I guess, really those are the only two characters that really needed to be developed, and Yuno was supposed to be a secret yandere until the end anyways