So I've seen a lot of ya'll discussing Sun's mean and sassy behaviour in the arts & crafts minigame, so i want to point something out: Sun and Moon were not originally in the daycare.
According to Tales from the Pizzaplex, the daycare attendant was originally a theater bot! They were eventually repurposed for the daycare, though we have little to no information about when or why this happened.
Suns behavior makes a lot more sense with this is mind. They were supposed to be a performer, not a caretaker! It's no surprise that they'd act this way when they weren't made for childcare in the first place! Sure they could've been reprogrammed, but we all know that fazbear entertainment employees absolutely suck at their jobs and likely didn't do it properly.
In conclusion: no matter how good they may have gotten at childcare, Sun is still a nitpicky, passive-aggressive, perfectionist performer at heart.