No, registration for JoSAA (Joint Seat Allocation Authority) is not mandatory if you only want to participate in JAC Delhi counselling (Joint Admission Committee, Delhi) and not interested in colleges outside Delhi.

JAC Delhi conducts a separate counselling process specifically for admissions to undergraduate engineering programs in Delhi government-funded universities. JoSAA focuses on admissions to various institutes across India.

You avoid the process of registering and filling choices for colleges outside Delhi, which might not be of interest to you. Focusing on a single counselling process (JAC Delhi) simplifies the admission process.

The decision ultimately depends on your priorities. If you are certain about pursuing engineering in a Delhi government-funded college, skipping JoSAA makes sense. But, if you are open to exploring options outside Delhi, registering for JoSAA might be beneficial.

I hope it helps!