But the Bismarck did get through. So did the Scharnhorst, the Gneisnau, etc. It was possible for a ship to sneak through, difficult, but possible.
The Graf Zepplin would not have been a good carrier. It was Germany's first design and no one got it right the first time. Just look at the Langely, Furious, and Hosho as examples. The heavy armament was a waste of space and tonnage (note that both the Japanese and Americans tried this and rejected it) and the aircraft were rather hurried adaptations of land based aircraft. Further, the compliment was very small for its size. The Zepplin might have done alright as a raider, but would have been horrible as a fleet carrier.