Is it bad if a mole itches really bad?

See your derm. Any mole that starts to itch, bleed or ulcerate should be shown to your friendly dermatologist.
Possibly. If a mole that was asymptomatic (no discomfort) is now itchy or changing in size and/or color, it must be looked at immediately to make sure it is not changing for something bad. It may very well be a benign thing such as seborrheic keratosis but without being looked it, it is hard to tell.