Unlike Yama , Brahma and Indra, I don't think Varun dev had any curse on him. Moreover if he had curse then how come people built a temple for him in Karachi. I think there is another temple for Varun dev in Kerela, but I might be wrong.
Given that he is lord of water it beats me why he isn't worshiped. Is it because he was beaten by Ravana so people don't think that high of him ?