There are two main reasons why Della misunderstands Jim's reaction.
First, Della expects Jim to act negatively about her decision to cut her hair. This is an important point, because our expectations exert a powerful force in our way of seeing the world. We know that Della is thinking this, because Della says to herself:
She looked at her reflection in the mirror long, carefully, and critically.
“If Jim doesn’t kill me,” she said to herself, “before he takes a second look at me, he’ll say I look like a Coney Island chorus girl. But what could I do—oh! what could I do with a dollar and eighty-seven cents?”
Second, when Jim finally comes home, he looks at her, and he gives a look that she never saw before. She is perplexed and does not know what this means. Here is what the text says:
Jim stopped inside the door, as immovable as a setter at the scent of quail. His eyes were fixed upon Della, and there was an expression in them that she could not read, and it terrified her. It was not anger, nor surprise, nor disapproval, nor horror, nor any of the sentiments that she had been prepared for. He simply stared at her fixedly with that peculiar expression on his face.
The text does not leave the reader guessing about why Jim gave an odd look. He bought combs for her hair. In light of this, Jim realized that Della could not use what he bought for her. Moreover, he knew how much Della loved her hair. In short, he saw in Della great sacrifice and love.
Here is what Jim finally says:
“Don’t make any mistake, Dell,” he said, “about me. I don’t think there’s anything in the way of a haircut or a shave or a shampoo that could make me like my girl any less. But if you’ll unwrap that package you may see why you had me going a while at first.”