Looks like you and Colin share the same criticism of the film.Well, I had a few laughs watching this movie. It had a lot of humour and therein lies the problem. It was the way the humour was derived that becomes an issue because it impacted the characterization of the main lead. I found it strange how Billy Batson as an actual kid acts more like what a 15 year old would really be like than when he is Shazam. The actor that played Batson as a teen plays him pretty straight and seriously. He actually did a better acting job than Levi did as the "adult" Shazam. It became problematic that Batson, as Shazam, turns into a complete 12 year old goofball while in his teen form he comes across as a somewhat troubled teen.
The film was entertaining but the characterization for Batson was pretty badly done as far as consistency goes. It was hard to believe that a fairly serious character as an actual teen would turn into a 12 year old moron in a spandex suit. From just the standpoint of humour, the film is probably a B, but if the spastic characterization is accounted for then the overall quality of the film is probably a C.