Personally, I am constantly distracted, albeit briefly and not necessarily negatively, when anyone I’m familiar with shows up on screen when I wasn’t aware that they were going to be in it. For example, Tommy Lee Jones in Captain America or Peter Dinklage in Infinity War. When Kevin Bacon showed up in First Class I shrieked giddily. As far as the main actors that I knew were going to be starring/appearing in any given MCU film, it makes no difference to me how big of a star they are and I see the actor unless they COMPLETELY disappear in the role. Brolin as Thanos, Ledger as Joker, I would even say Gary Oldman as Gordon, just to give a few comic book movie examples. But anyways, I guess if the films were FILLED with unknowns then it would be jarring to have a big star show up, but as it stands it doesn’t bother me.