Atheist here, but grew up mormon: Jesus is definitely the main character. I’m hazy on the details but it’s even mormon canon that a lot of events in the old testament involved Jesus in pre-birth spirit form (or something idk), or were at least analogous to his life. Joseph Smith and Brigham Young are to Mormons as any notable dead pope is to Catholics - revered, spiritual authorities, allegedly could talk to god, but they’re not worshiped and they didn’t come up every Sunday, they were kind of in the background

They revere Joseph Smith for sure (and some other made up folks like Moroni or whatever) but I think it’s incorrect to say Jesus has taken a back seat in their faith. I just toured one of their temples (which they allow only briefly before they consecrate it), JC was everywhere. Not sure I saw any imagery of Joseph Smith, but it was a lot to take in and I may have missed it.