I'm not going to say that Skarsgard looks like my idea of Flagg brought to life, although I imagine I'll enjoy him in the role more than Sheridan. As far as smiling goes, though, I'm concerned less with Flagg smiling enough than with him serving as a representation of seductive evil, and in doing so feeling dangerous like he felt in the book. I mentioned Billy Hargrove, but Chris Hemsworth's character in Bad Times at the El Royale had that quality of being charismatic and at times even superficially nice, but you could still tell he was dangerous. I see Flagg as someone who some people would be drawn to but others would recognize as trouble right away, and that's not the easiest line to walk for an actor.
I don't know if Skarsgard can manage that, but ultimately I'd rather have a Flagg I can take seriously than one who just seems like a clown. Incidentally, I mentioned before (elsewhere?) that I liked Aaron Taylor-Johnson for the role, and that was before I saw him in Noctural Animals. He would be genuinely frightening as Flagg.