Is Susie an unbiased narrator? Does she soften the impact of her mother's infidelity, etc.?
Yes, she does understand more than a regular first person narrator would understand, but she is in no way unbiased. She tries to be understanding, and she generally is, but she does have an opinion on who is doing what and how that effects everyone else. For example, she sometimes sends "prayers down to earth," in hoping for a certain outcome for an event. This happens when she sees Artie walk up to her sister, Lindsey: "'Catch her,' I said. A prayer going down to Earth" (122). Unfortunately Lindsey wants to remain aloof, and the narrator says "Lonely...on Earth as it is in heaven." She definitely has an investment in what happens to her family.
Because Susie is dead, her first person narration is really closer to third person omniscient. Susie can report all perspectives because she has access to their innermost thoughts, as well as their actions.
In this way, it does soften the impact of her mother's infidelity because we can read her thoughts, and see that nothing is black and white.