Cake batter should have a "dropping" consistency; this looks like the batter dripping smoothly and slowly from a spoon when tilted.

Does it matter if cake mix is runny?

Thick cake batters results in a lighter fluffier cake and the thin soupy batter would be dense and heavy. I find thick batter will result in dense, heavy cakes. Watery batter results in light and fluffy cakes - that's from my experience. I love watery batter because my cakes are more moist compared to a thick batter.

What happens if cake batter is runny?

In the case of chocolate cakes, a runny, thin batter is okay because the leavening of your cake relies mostly on chemical leavening agents such as baking soda and vinegar. It is the same case for chocolate cupcakes.