If a toy car moves down a ramp, would the speed be constant? When it moves down a declining ramp, would the speed of the toy car be constant? If not, what would the distance/time graph look like? (distance as in the distance the car moved)

No. A car going down a slope experiences an acceleration of $g \cos{\theta}$, where $g$ is the strength of the near-constant gravitational field on Earth and $\theta$ is the angle of the slope. By integration the car must have a speed of $gt \cos{\theta}$. By integration once more (assuming the car starts with no velocity and at the top of the ramp) the car must be displaced by $\frac{1}{2} gt^2 \cos{\theta}$.

The graph of distance versus time would look like a parabola.