A 'below average' person, in the holistic sense, rarely gets recommended. However, a student who is 'below average' academically can definitely get recommended, provided there is enough substance in what they have done in life if not studies.
If you are below average in academics and do not have anything else to show either, then do not expect much. Then it reflects on your attitude more than anything else. What have you been doing all these years?
You must also think about your definition of being 'below average.' I had 79% in 10th, and 75% in 12th. When I got recommended for NDA, I was in my second year of Engineering. It was my 6th SSB attempt. I had 3 backlogs already in the first 2 semesters. The Interviewing Officer during my SSB knew about it. What would you call me – average, below average, or a weak student? I didn't have any significant extracurricular achievements or participation either.
Sometimes it's the circumstances from your life which decide if you are average, below average, good, or weak. For example, consider the son of a hand-rickshaw puller who gets 60% in 10th studying in a good English Medium CBSE School. What is he? For me, he is a fighter with a great attitude. But if I scored the same 60% coming from the background I had, I would call myself weak.
Ultimately, the SSB is looking for potential and a positive, resilient attitude. Your academic record is just one part of the much larger picture they assess. Focus on showcasing your overall personality, your learning from life experiences, and your genuine motivation.