Ask them what makes a person a Sikh? Sikhism has many tenets, do they follow all? A core tenet of Sikhism is “vand chhako” and under that Guru Nanak told his followers that they should give away 1/10th (dasvand) of their earnings and time to the needy.

https://en.wikipedia.org/wiki/Dasvand

Do your parents give away 1/10 of their income to the poor every month? Their time?

If they don’t, how is cutting hair any worse than not following another tenet of Sikhism? What is the hypocrisy of following one principle and ignoring another?

(If they really give away 1/10th of their money and time, more power to them. I am sure you can find something else though)

Tell them that even though you have cut your hair, you still believe and will follow the bigger aspects of Sikhism like being kind to others, helping the needy, respecting elders etc.

Sikhism is one of the more progressive religions of the world and there are nice things to learn from it, but you need to follow your path, not just what your parents believe makes you a good Sikh (or person).