Thapar University takes admission on the basis of JEE Main ranks. For Mechanical engineering your JEE main rank has to be under 42K (ideally) in the last round for general category and outside Punjab students.
70% seats are filled through JEE Main ranks and the rest 30% through board PCM merit. And the for admission through Board merit, the marks required are on the higher side (88% to 95%). So, with your marks the chances are difficult.
Hope this helps. Good luck.