Technically you could use it. However, it may not turn out the way you want it to. For instance, if you’re using a powder makeup highlighter without a foundation, it wouldn’t adhere to your skin. It’ll probably go away the first time your rest your cheeks on your palm. Besides, if you have dry skin, this method will accentuate flakiness. Therefore, it’s best to refrain from using highlighter without a foundation. Preferably, there has to be some base product on your skin before you apply a highlighter. There are alternatives available for foundation, you can opt for those if you don’t like applying foundation.