In character with Della, her fears are not selfish. She wants to look pretty, not for herself, but for Jim. For, after she returns home, she uses the curling iron on her hair to make herself look the best that she can for her husband: "If Jim doesn't kill me....But what could I do?" As she hears his footsteps, she prays, "Please, God, make him think I am still pretty" since she does not want to displease her husband, whom she loves dearly, obviously.
When he enters and stares at her "fixedly with a peculiar expression," it is not anger, surprise, disapproval, or horror, write O. Henry, so Della need not worry about her looks. Rather, she is again concerned with Jim's displeasure. If she were vain, she probably would not cut her hair; then, even if she were to cut it, she would quickly say something in reaction to Jim that demonstrates her resentment of having cut her hair. But, in this delightful story about unselfish love, Della simply tries to make amends: "It'll grow out again--you won't mind, will you?" Della does not speak of herself; she is not concerned with her looks, per se as some readers mistakenly suggest. She is only concerned about Jim's happiness, and he has been happy having her with long hair; her unselfish concern is consistent with her character from the beginning of O. Henry's story.
How else could Della be, as O.Henry declares, "the wisest...the magi"?
What Della is afraid of is that Jim will think she looks ugly and (presumably) that he will not love her anymore.
We can see this in a bunch of ways. First, she spends 40 minutes trying to fix her hair in some way that she thinks will look nice. Then she says a prayer that he will still think she's pretty. Then, when he comes home, she asks him about it. She says
Don't you like me just as well, anyhow? I'm me without my hair, ain't I?
It is pretty clear that Della is worried about her looks. Her hair has always been her pride and joy and now she has cut it off. No wonder she is worried about what her husband will think.