What matters first and foremost is character likeablility; as long as most of the cast is likable enough, the anime can be good—if they aren't likable at all, then it'll be bad regardless of how good the story is.
But complex characters can elevate an already-good cast (and therefore the whole anime). For example, FMAB wouldn't have been nearly as good if it weren't for the more complex characters like Hohenheim, Mustang, Scar, Bradley, and Envy. And one of the biggest triumphs of Attack on Titan: The Final Season is Eren's character development. Mob Pyscho 100, on the other hand, is about as good as a show can get with just simple and likable characters.