It is okay to use liquid measuring devices for dry ingredients. Liquid and dry measuring devices measure volume, which is the standard form of measurement for most baking recipes in the United States. A teaspoon of flour has same volume as a teaspoon of water, for example.
However, liquid and dry measuring devices are designed to accommodate their respective ingredients and yield precise amounts. Accurately measuring dry ingredients by volume requires filling a dry measuring cup to well over its brim and gently shaving off any excess with the back of a knife. The shapes of liquid measuring devices such as graduated jugs do not allow one to level dry ingredients. Inaccurate measurements may yield undesirable results.