Do I have to rotate the camera to top-down view if I want to crate a top-down 2D game or just disable gravity?
You can do whatever you want.
That said, it is worth considering standards. Picture your game space as a 2D grid. If you retain the default camera angle, you would be working along the X and Y axis. If you rotate your camera to a down-facing position, you would be working along the X and Z axis.
I would say "leave the camera the way it is"; it is far more conventional to refer to a position on a 2D plane as "X, Y". While you could easily portray coordinates as "X, Y" to the user, you still have to work in "X, Z", yourself. Any built in methods that apply to two-dimensional coordinates will also assume use of "X, Y", including the directional vectors housed in the
Why add another layer of complexity to your project? Use the "X, Y" plane, and stick to 2D standards.