Indeed, if it wasn't flat it would not perfectly cancel.
The drop off applies because the force of gravity is proportional to 1 / distance^2, so in this case the napkin would still have infinite mass, but it would not have infinite density, so if you took a surface integral of the gravitational force provided by each point in the napkin, over the whole napkin, it would converge on a finite value, as each point contributes less and less force as you get father away.
The drop off applies because the force of gravity is proportional to 1 / distance^2, so in this case the napkin would still have infinite mass, but it would not have infinite density, so if you took a surface integral of the gravitational force provided by each point in the napkin, over the whole napkin, it would converge on a finite value, as each point contributes less and less force as you get father away.