Nope.
Mom was wrong, at least for one Donald Unger.
Behold one of the glorious winners of the 2009 Ig Nobel prize!!
Arthritis and Rheumatism, vol 41, p 949
http://www3.interscience.wiley.com/journal/86510619/abstractDonald Unger, an allergist in Thousand Oaks, California, earned the medicine prize for addressing another timeless question: does cracking knuckles really cause arthritis, as his mother warned him it would?
To resolve the issue Unger embarked on a long-term controlled experiment, and began cracking the knuckles on his left hand twice a day, but not those on his right. He has done so for more than 60 years, and never suffered arthritis in either hand.