Desi's mom, Jacqueline, is a huge clue that something is super wrong with her son. For one thing, when she first met Amy, she whispered good luck in her ear and "coldly" (11.126) kissed her cheek. We get that meeting your son's girlfriend is pretty awkward, but what's that about? We've also already mentioned that Amy looks exactly like her. Cue the Psycho theme.
We never get a clear-cut answer on this, but it's almost like Jacqueline is jealous of Amy. When Nick visits Desi to try to get answers about his wife's disappearance, she sarcastically tells Nick that Desi "'can't stop thinking about the superb Amy Elliott'" (21.111)—and later, when the subject of the letters Desi wrote to Amy comes up, she asks him if he kept all the ones she sent him (21.122). Burn, Mother.
To top it all off, she seems overly protective of Desi, telling Nick to call "our lawyer" (21.132) next time he has questions for Desi. Isn't it kind of odd that Desi's in his mid-thirties, lives with his mom, and doesn't have a lawyer of his own? Talk about needing to cut the apron strings.
Ultimately Jacqueline's unnaturally clingy, controlling relationship is her character's key attribute. Amy herself seemed to recognize this while she and Desi were dating—so in classic Amy fashion, she scratched her own face and told Desi that his mom picked a fight with her. "The woman was so possessive, and so cold to me, she might as well have" (50.5) attacked Amy for real, Amy explains.
Of course, this can't exactly make Jacqueline have more respect for Amy, and framing Desi for her kidnapping doesn't help either. We last see Jacqueline being escorted out of the police station for accusing Amy of Desi's murder (which she is guilty of, by the way).
Bottom line: if Amy had changed the course of history and married Desi, Jacqueline would have been the ultimate Monster-In-Law. We're positive that no woman is good enough for her little boy—except, maybe, Jacqueline herself.