The basic problem with this movie is it seems like the director and the studio thought “this is a scary movie for junior high school boys” and what’s worse, they have very low opinions of the tastes of junior high school boys. It’s also extraordinarily sexist as I’ll write in detail below.
The manga had very complete and mature characters and plotting from the start, but these guys were all turned into thirsty idiots who are too busy having sex to realize there are Titans right next to them.
The major themes of the story, like family love and sacrifice, and political class conflict, were completely absent. All that was left was an army, and it was the world’s most disorganized army.