Re:Cutting diet & cardio
If you're loosing fat and maintaining muscle without cardio then keep at it, why would you need to add cardio if this was the case?
So no it's not essential, it's optional. You may look worse as you're more depleted an thus your muscle look flat/soft. This is just water/glycogen being low. However, too much cardio can cause muscle loss due to increased cortisol altho obviously i can't say for certain which you experienced. When you added cardio did you loose strength?