In principle, yes, a wind tunnel would help you create a car that is either faster or more efficient, or to improve the ground effect, or let you control it bet

In principle, yes, a wind tunnel would help you create a car that is either faster or more efficient, or to improve the ground effect, or let you control it better. It does that by letting you check the places where the airflow becomes chaotic, that can't be simulated very well. On practice I doubt you are trying to optimize a tiny car. And the chaotic flow is very dependent of scale. So the only parts that transfer from a tiny car into a normal one are the ones that are easy to simulate.