Kaguya Otsutsuki is a very problematic character in the series. There were so many wrong turns that it's almost unfathomable. Some chalk it up to her appearing out of no where, while others say her goals weren't properly explained or expanded upon.
However, I believe the worst part of Kaguya's character is her mentality. She is supposed to be the Goddess of the Narutoverse, yet she is easily the most incompetent character in the entire series. She couldn't speak or think for herself, she constantly followed orders from Black Zetsu, she made really bad mistakes, and she shows almost no emotion at all. For someone of her status, why was she portrayed as so... "simple"? Here are three questions for you to ponder-
1. Why do you think Kaguya was like this? Why can she not do anything by herself?
2. Do you think she was always like this, or did something change her?
3. Why do you think Kishimoto decided to portray her in this manner?
I think the beginning of Kaguya's character development in 681 (when she was crying) initially looked like it was heading in the right direction, but I think time constraints kept Kishimoto from writing a good, solid character. There was potential to create a good character, but just either not enough time or Kishimoto did not have the talent to write another well-fleshed out character.