This is a difficult argument to make. Jim is an honorable character in the short story. He sacrifices for Della and clearly loves her. But since this is an exercise is critical thinking, we can say that Jim is the antagonist in a certain way.
He does come off superficial in one statement that Della makes. When Della cut off her hair, she worries about what Jim will think. She says to herself:
"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?"
That Della would even imagine this shows that Jim might be superficial by caring about Della's appearance too much. The implication is that Jim might care about her looks without really caring about why she did what she did.
Second, we might argue that Jim should have worked harder to support Della. Could their poverty be a reflection on Jim?