Yes it is perfectly fine to appear for only JEE Main 2nd exam that  is JEE main April without appearing for JEE main January edition because you can appear for either of them or both of them those who appeared for both of them the best of the to normalise score will be counted for the rank calculation therefore even if you don't appear for JEE mains January session and if you perform well in the JEE main April edition then you are going at no loss.