This is anecdotal, but I find that my Sims paint nothing but masterpieces all day long until they reach that part of the aspiration and then....
...nothing. LOL
I do things like "Okay, do some brain-boosting yoga, then cloudgaze, then take a thoughtful shower.... and then paint." I think it helps? but don't have real evidence to back that up.
I see the notification "Congratulations! Sim 1 has just finished painting a masterpiece!" but then realize it was a figure drawing. A figure drawing that doesn't count for the masterpiece part of the aspiration.
