Depends on what type of turtle it is
You have sliders, right? Sliders can have shells that are more domed than other turtles... all mine are 'domed' to a degree, but I've seen pictures where their shells are very flat. It also depends on the turtle, too
I have a southern painted and midland. I've seen that southern shells tend to be a bit more 'domed' than midland shells, which seem to usually be flatter on top.