I doubt people like Michael Keaton, Heath Ledger or Ben Affleck are considered mediocre actors and fans lashed out against their casting nonetheless. The iconography of certain characters, particularly those who have been ingrained in popular culture, isn't and shouldn't be seen as unimportant aspect that can be changed all the time so it makes all the sense in the world for fans of those characters to care about said aspect. But I truly doubt they would rather have a mediocre actor who looks like the character over a great actor who doesn't. This isn't something that's mutually exclusive anyway, studios could very well end up casting mediocre actors who don't resemble the characters at all. I've never seen Zoe Kravitz or Sasha Calle in anything so I can't speak if they're talented or not, my opinion of their acting would be based on what I see them do as Selina and Kara. I've seen Jeffrey Wright though and I know he's a good actor so him playing Gordon doesn't bother me at all. But in general it's a crappy thing to do to attack fans for wanting characters they love to be translated more faithfully to screen. This notion that people who want movie Superman to look like comic Superman makes them shallow or prejudiced should stop.