if you want to start a fear frenzy, i recommend sending your sims to von haunt manor at night. i had them look at the paintings a few times, and within minutes mimsy went and possessed something and freaked them all out. i've also found that anytime the scared moodlet is overridden, you can have them go look at the painting repeatedly, which causes mimsy to possess something again (making the fear/screaming increase).
in the end i got my sims up to a scared +3 moodlet that didn't fade until mimsy left in the morning.
https://i.imgur.com/ABxZKAV.png