It is wrong to pretend to have a mental illness.
Karen is burned out from work, and wants to take an extended vacation, but doesn't have any vacation days.
Karen wants to give her boss a reason to get a vacation.
Karen tells her boss that she's simply at the edge of her rope, and needs some time off to reset her focus and priorities.
Karen tell her boss that she's been seeing a psychologist for bipolar disorder, and needs a couple of weeks to adapt to her medications without the pressures of work.
Karen's boss is able to respond to her honest claims, and Karen is able to relax with the knowledge that she was honest.
Karen's boss gains a false impression of her, and Karen has to spend the rest of her career nurturing a lie.