This would be bad enough had she not had long-term exposure to a boy-crazy idiot named Molly Thomas and her man-hungry lunatic step-mother Connie Poirier. We know that she was paying attention when Connie left town in despair because she couldn't find a MAAAAAAYYYYYYYUUUUUUUUUNNNNNNNN and only returned when she'd got one so she was primed to think that having male companionship of any sort was all that really matters. Watching Molly mope because she was separated from her cute boy and watching her leave town because Connie drove the new cute boy away sealed her teal-and-lavender doom.
This means that most of the irritating screeching that baffles and angers her parents about how no one could love someone with a stoopid, ugly face is the direct result of her insane fear that if she didn't have a cute boy, she was destined to die alone and unloved and worthless. It also explains her messed up academic priorities as friends, teachers, coursework, self-respect and extracurrical activities could and should be sacrificed on the heathen altar of attracting male attention so that she could be the girl in the white dress everyone wanted to see. It matters not that there's a whole life to be lived after the great big party where she's the one everyone looks at, she's gotta look for a man.
Not, of course, that she's interested in the men she acquires. Unlike her ugly brother, she's not hampered by anything like a sense of morality gone wrong because she sees herself as the star of a sitcom. Mike might actually love Deanna for herself because she doesn't present a threat to him or make him fear humiliation and loss. Liz loves the attention she gets and the status she is endowed with, not the person doing the endowing.