Shazam wasn't a "hit" like some DCEU die hards say and it wasn't a failure or flop either. But it didn't do great.
It has nothing to do with campiness and I agree with Boy Scout that people don't understand the term "camp"
I think it was the marketing which was just meh. When you have a C list hero/brand like Shazam you gotta do it big with the marketing. Especially when you don't have a MCU name recognition backing him/her.
You could have a lot of fun with the marketing when the main hero is a kid in an adult body. And when it's a comedy. But they didn't do anything special. They could've gotten very funny and very creative with the trailers, posters, billboards, social media. I mean the main characters are teens and they didn't do anything cool with social media. Did Billy Batson have a Twitter or did they do any viral marketing through YouTube? They could've made the YT account that Freddie has in the movie real and had various clips of "Captain Sparkle Fingers" in action. Or you remember those little vignettes of Loki talking to kids? You could've done something similar with Zach Levi in character going to a high school class.
I also don't think the quality of the movie was that special. Not saying it was bad or anywhere near close to that, but it wasn't anything that demands people to go see it in theaters. And, this is anecdotal, but I dont know anyone who saw the movie who LOVED it like how people really enjoyed other movies. Then again, I could say the same thing about huge hits like Jurassic World 2, Aquaman, and Captain Marvel.
Anyway, yeah I just think it was a bad release date and very poor marketing. It should've done more