She’s your secretary, she’s paid to be pleasant.
Your cynical view of reality is also extremely narrow and misleading. The world is more varied and colorful than you think.
She would have laughed you heartily for suggesting that she could simply wear different personalities to her work like clothes. This is precisely what highly emotional people struggle to achieve. On the rare occasion when she feels dejected or depressed (the kids, twins, are 16 and sometimes a handful), everyone around will definitely know.