Thanks to Nutella for bringing my attention to this. It's clear that I need to remove Playful as the Ideal Mood for Mischief as it doesn't affect the Skill in any way (not even effective level), but it doesn't appear that Angry is really the best route for Sims to level the Skill. The post by
@DarkWalker indicating Skill gains from various Emotions lists all Anger states as producing a +20% Skill gain for Mischief. Happy increases all Skill gains by 20% for regular Happy and 40% for Very Happy, same with Very Confident. You'd want to be in one of these two in order to level it faster than by being Angry. I don't have time to directly test that data at the moment and give a comparison, but doubt it's changed.
While Enraged Sims get a +3 boost to effective level (a dangerous emotional state). Regular angry gives +1, but being Happy gives the same bonus for plain 'Happy' but +2 to ALL skills at Very Happy. I'm not sure effective level is all that important for Mischief, but again it's not necessarily good to be enraged just to gain Skill.
This is a somewhat complicated one anyway, because if you have enough Happy Moodlets, getting a Sim angry can be a challenge. It's certainly doable with the Angry paintings, but much easier to just make a Sim Happy to get the same boost. I've got some editing to do on this guide and I'm sorry for overlooking this data that was posted some time ago. Playful just seemed natural when initially working on that page, given the pranks etc. and I completely missed the mark with this one.
If you get a Sim Playful and had Happy moodlets, you lose the skill boost to Mischief entirely. A beautifully decorated room with some other +Happiness Moodlets on the Sim is the route I'd go to raise the Skill. I'm making edits to the live page now so that people get the right info.