I completed my diploma in mechanical engineering this year. Can i fill NCHMCT JEE next year??

Hello,
You can apply this year also. But if you want to apply for next year then yes you can. JEE gives 3 attempts to each candidates and every candidate can attempts both the exams conducted by NTA.
Both will be considered as one attempt only.
So if you want to appear this year then you can apply for the April month. It's better to give a shot and you have time to study also. You can prepare well and appear it.
You will get lateral entry if you join any government college or any private Institute. If you score good and get an admission in any NIT campus or IIT campus then you will get admission from 1st year only.
Hope it helps you!