Is it bad for my 9 month old's posture when she's sitting on the floor with her head bowed down to play with toys for long periods of time?

For a long period of time it will be bad for her posture as it will cultivate a habit for her to bow her head down to play. Like what Diana mention, get a table and chair for her to sit and play with. As she is still quite young to be sitting on a chair on her own, get an adult to stay by her side to play with. You can also place her in an excersaucer so she can play toys at her chest level.