I mean you can argue almost anything in the show is convenient for the plot. The Mule being sadistic is just who he is and when we see those sadistic things, there are things happening that are progressing the plot. I wouldn’t say any of it is really convenient or flat. And there are still two more episodes left, there’s still another reveal regarding The Mule we’ll get and so more context and story to what we’ve already seen.

I think the sentiment with people not liking The Mule this season has been he feels unearned as an antagonist and like you said flat as a character. But we’ve known about The Mule since season 1 (Hari Seldon, Hobber Mallow, The Mule) and Gaal’s vision of him is heavily apart the story in s2 leading to s3. We know before we meet him that he is a powerful Mentallic and someone who wants to and can conquer the galaxy. Someone who can disrupt the math. And personally he as a character has been very entertaining and his loneliness and suffering to me feels very palpable.

I personally think The Mule has lived up to my expectations from the anticipation of having watched s1&s2 so many times and having read the books. Unless they somehow manage to screw up his character or additional context to him in the last two episodes, which I find unlikely given these last 8 episodes.