There are a few factors that can cause your cilantro lime dressing to turn out bitter. First, be sure you are using only the cilantro leaves. The stems can cause the dressing to taste bitter. Similarly, you will want to avoid getting the pith of the lime in your blender pitcher. Your choice of oil can also affect the flavor. I recommend light sesame oil for this dressing. Olive oil works in its place, however if you are using extra virgin olive oil, it may increase the bitterness of the dish.