This is a tough question that has no clear answer to it. The Mitsuki Disappearance Arc is an anime-exclusive one that is not in the manga. In this way, it is, technically, not essential viewing. However, I think it adds a lot to his character in a way that only enhances the Boruto story. It is relatively long, though, for Boruto filler, so I, personally, skipped through several scenes to get to the good stuff. I recommend doing the same to most viewers.