For the most part, the author remains detached, and through writing down the simple thoughts and feelings of Loisel, allows the readers to decide for themselves how they feel about Loisel. If you read closely, you can not a tone of sarcasm however, a tone of insult, one of dislike towards the woman. Take for example, this passage:
"She had no dowry, no expectations, no way of being known, understood, loved, married by any rich and distinguished man; so she let herself be married to a little clerk."
Here we see Maupassant exaggerating Loisel's "misfortunes" a bit; his straightforward telling of her thoughts, that she felt she had absolutely nothing, and that absolutely no one loved her, makes her sound petty, whiney, and childish. He could have said, "she had few expectations, a small dowry, was not every well known, understood or loved," but instead he uses the dramatic "no, none" to describe her plight. This exaggeration is probably not true; however, Maupassant's describing it that way reflects how Loisel feels about it, and reveals her petty nature. She is like a child throwing a temper tantrum; Maupassant doesn't soften her petulant attitude one bit, but tells it exactly how she feels it, which is in selfish and self-pitying terms.
So, even though Maupassant appears to stay neutral by revealing her thoughts as they are, it is that open and honest revelation of her selfishness that helps us to see her as a selfish and ungrateful woman. It's hard not to feel that the author felt the same way about her, and used that tactic to reveal her childishness. I hope that those thoughts helped; good luck!