"Good artists copy; great artists steal". It's a fairly well-known saying that speaks to the way art evolves over time as the innovations of influential artists seep into the work of those who come along later, but it's worth breaking down what exactly the phrase means by "steal". Mimicry or simple replication isn't enough; you have to make something your own in order to steal it, add your own unique spin or use it in a particular way that stamps your name on it, and that means that I, Tonya - a biopic whose style was quite clearly heavily influenced by the work of Martin Scorsese - doesn't qualify as an act of theft. Appropriately then, it also doesn't qualify as great art - merely quite good.
At the very least it's a vast improvement over the pale Scorsese imitation that David O. Russell has been doing for the last few years, thanks in large part to director Craig Gillespie's much firmer grasp of how to make the particulars of this style - such as the fourth-wall breaking narration, or the eclectic soundtrack - work on-screen. But just as important to why I, Tonya works where films like Joy don't is the simple fact that the story of disgraced American figure skater Tonya Harding is both a) actually worth telling and b) well-suited to this style of film-making, hitting all the required funny, sad and tense beats as it focuses on a number of vibrant, almost larger-than-life characters who you actually want to learn more about and see interact with one another.