There are several possibilities for why your plant’s leaves are curling and yellowing. Improper watering, too little or too much, is often the main culprit. Tomatillos need to be watered regularly and deeply (1” weekly), ideally in the morning so leaves can dry before nightfall. Additionally apply 2-3” of mulch around plants to conserve moisture. I cannot really tell from the photos but another possibility is a mosaic virus or blight, for these symptoms include curled leaves and yellow discoloration on the leaves between the veins. There is no treatment aside from removing the affected plant. Here are two links to growing tomatillos you might find useful and possibly further pinpoint the problem if the possibilities above don't seem to fit – Utah State and UMN Extension. Good luck!