My hypothesis is that a car with heavier mass experiences larger friction on the ramp and with the floor surface
The friction force depends on normal force magnitude which depends on mass, but the mass cancels out in the equation of motion. For the car going downwards the equation of motion is
$$\underbrace{m a}_{F_\text{net}} = \underbrace{m g \sin\theta}_{F_g} - \underbrace{\mu m g \cos\theta}_{F_f}$$
where $a$ is the downward acceleration magnitude along the ramp, and $\mu$ is coefficient of friction which is very small for a rolling wheel. Notice how car's mass appears from both sides of the equation, hence
$$\boxed{a = g (\sin\theta - \mu \cos\theta)}$$
This shows that both cars have the same downward acceleration, i.e. car velocity at the bottom of the ramp does not depend on its mass. At least not theoretically.
I conducted this experiment at school and found the car with heavier mass traveled slightly less than a car with lighter mass. I was wondering if this a valid result or if some added friction of some sort is skewing my results?
There will always be some margin of error. The equation of motion predicts that both cars will reach the same horizontal distance after leaving the ramp. However, there are many effects that we have not considered here, such as air resistance which depends on car's geometry etc. All these add up to measurements uncertainty.
Check similar question here:
Does mass affect distance travelled by a toy car?
