Mr. and Mrs. Aarons are Jess's parents, and they're both kind of ciphers, or blanks. We don't get a sense of knowing them that well. In fact, you might realize that we never even learn their first names. This puts them in marked contrast to Leslie's parents, who are identified to us most often as Bill and Judy – on a first-name basis. This lack of first names makes Mr. and Mrs. Aarons seem almost like archetypal figures, or as only parents – we see them most often described as "mom" and "dad" – and less like individuals.
Because we see them through Jess's eyes, which show us neglect and absence – the kinds of flaws we're quick to hold on to – it's harder to see their good qualities or their love for Jess, compared to how they treat his sisters. It seems like Jess never gets the love or acknowledgement he craves from his parents. It isn't until Leslie's death that we see their love for him, or see them pay him specific attention.
Most of all, Mr. and Mrs. Aarons seem kind of old-fashioned and stereotypical in their ideas about how boys and girls should behave, and Jess has really taken on some of his dad's ideas about what it means to be "a man" versus a little girl or a baby. Maybe, though, we owe Mr. and Mrs. Aarons some consideration: there is goodness, tenderness, and care beyond Mr. Aarons' strict façade and Mrs. Aarons' stressful budgeting, as we certainly see by the end.