It's completely fair.
Regardless of whether or not you enjoyed Man of Steel, it isn't a well-constructed film from a writing standpoint. That is what critics care about the most. Regardless of how fun the action is, if the story isn't well-paced, they aren't going to like it. Compare Man of Steel to Superman Returns. The plot is better constructed, better balanced, and you understand the characters better.
Superman Returns is boring, but technically it works.
Man of Steel is awkwardly passed, imbalanced, and has a lot of problems. It's fun, but it's flawed. Critics will tear it apart. The fact it's directed by Snyder doesn't help.