I am a Diploma student (in Computer Engineering), and I also appeared for JEE Mains but not in CUET. Am I allowed to register for CUET?

Yes, Diploma holders are generally eligible to apply for CUET (Common University Entrance Test). Specifically, a three-year Diploma in engineering or equivalent, recognised by AICTE or a state board, with at least 45% marks in aggregate, is considered a valid qualification, according to Toprankers.