It can be if you are doing it to much or to hard. Really only a chiropractor should be adjusting you. Doing it to often can cause abnormal wear and tear on the joints, as well as you can strain surrounding muscles. Try to cut back to once a day if you can. : ) Instead if you are achey try apply a hot wrung out towel and placing it around your neck, it will allow more blood flow to the area and relax it without the popping.
I crack my neck, quite frequently. In fact I would rather crack my own neck than even a chiropractor no matter how experienced. (Paranoia that s/he will BREAK my neck) It could gross a lot of people out and may not be an appropriate activity under certain situations. So I wouldn't worry about it. (Maybe I should be named Crackerneck or Crackerknuckles instead of Crackercorn)
Neck cracking isn't a good habit and you should stop as soon as you can. The harmless popping of gas you hear when you crack it is as a result of putting a lot of stress into the joint, which wears the joint down and the ligaments. The other neck cracking side effect is that it can lead to stroke, especially in people under 60 years of age. Read on from www.webmd.com/stroke/news/20030512/neck-cracking-raises-stroke-risk.