Does refusing to cut long hair constitute disobedience to one's mother or a sin?

You must shorten your hair in obedience to your mother, because honoring one's parents is an act of great reward. Note that growing hair long for men is a customary practice, not a religious requirement. Obedience to parents is obligatory as long as it does not involve disobedience to Allah, and as long as they have a valid purpose, and it does not cause harm to the child.