No such thing as too much sun for a Zinnia. Too much water, probably in combination with finely-textured soil.

Thanks purpleinopp! Whats a good watering schedule for zinnias in containers? Is 2-3 times a week good enough?

Unless the weather's the same every day, a set schedule might not be possible. Water when the pot feels significantly lighter and it has mostly (but not completely) dried out.

What kind of potting mix is that?

It's the Supersoil potting soil