Dear Aspirant,

     I'm here to help you regarding your query !!

    Mop-up round is something which happens after round 1 and round 2 for all those students who haven't got a seat in any college of their choice before, during this session they'll allot the left over seats based on your rank which will occur in offline mode but for some exams like AIIMS it takes place in online mode. The seat allotment is processed by respective states against the reverted seats from 15% through all India quota. And yes, you can have ghe chances of getting a seat eventhough if you have got less marks if many seats are left. 

 Hope it helps!

 Best of luck !