Did the Germans really blunder at D- Day?

It isn't as simple as that. Rommel wanted all the forces up close on the beaches, figuring air interdiction would stop counterattacks. This was true. On the other hand, von Runstedt wanted to keep most of the forces off the beaches, figuring naval gunfire would be too destructive. That was true, too. Hitler tried a compromise, neither fish nor fowl, and probably worse that either of the two bad alternatives, but they were really between Scylla and Charybdis in planning the defense, especially since the threat was a lot less than the Red Army rolling across eastern Europe.