What is wrong with my swiss chard?

Hard to tell, I don't grow swiss chard, but it is just a perennial beet. I think I see a few skeletonized spots on the leaf in you photo. leaf miners do attack beets so that is a possibility.

Sure looks like leaf miners.

Concur with above. I get the same damage on spring greens when the weather warms. Neem seems to work on them, so long as I get them early and consistently. They're a bit of a tougher nut since the damage usually appears later (closer to harvest) when I really don't want to use anything more intense. I've seen a few recommendations for Bt sprays for them, which I'll have to give a try.