It's important to season the kale pesto well to mask the bitter flavor of raw kale. If you find the pesto tastes bitter or too vegetal, add more nutritional yeast, salt, and pine nuts. Avoid adding the kale stems to the pesto, using just the leaves. You could also use baby kale instead of regular kale. Baby kale is just kale that has been harvested early in the growth process, and it has a more neutral flavor than mature kale.