Can an average student with ordinary academic marks crack the UPSC exam?

Yes, UPSC is a great equalizer. Past academic performance is irrelevant because the exam purely evaluates your current knowledge, analytical depth, and consistency. Success depends on discipline and strategy rather than a high GPA or elite college pedigree. Once you enter the exam hall, everyone starts from zero.
Real-life examples of "average" students who succeeded:

Anju Sharma (IAS): She failed her 10th-grade Economics pre-boards and 12th-grade Chemistry exam. She learned from her failures, cleared the UPSC on her first attempt at age 22, and is now a senior officer.
Awanish Sharan (IAS): He scored a modest 44.5% in Class 10 and 65% in Class 12. Despite these "ordinary" marks, he showed that persistent effort outweighs early academic struggles.
Muthyala Raju Revu (IAS): He actually failed his 10th-grade English exam. He eventually topped the entire UPSC exam (AIR 1) in 2007.