Is it true that the "terrible two's" are even more terrible if mothers smoked during pregnancy?

No. The "terrible twos" are not terrible; they are a wonderful time with more learning, exploring, and fun than just about any other time in kids' lives. This time can be challenging and energy draining for parents, thus the name; but how "terrible" it actually winds up being depends on the attitude with which you approach it. Try to see things through your kid's eyes, and laugh all the time.
Yes. Several new studies show that toddlers of women who smoke during pregnancy have an increase in negative behavior such as rebelliousness, compared to toddlers whose mothers did not smoke. No one yet understands why, several theories have been proposed. Now we know that smoking while pregnant affects both body growth and brain development. If you smoke ask your doctor to help you cut back or quit.