Can I file an RTI application to know why my government job application was rejected?

Yes, you can file an RTI application with the concerned public authority (the department or organisation that conducted the recruitment) asking for information such as your marks/scores, the selection criteria applied, the cut-off for selection, and, where relevant, comparative information about the process that led to your non-selection.

Public authorities are generally required to respond within 30 days, and if the information genuinely exists in a form that can be disclosed (some purely deliberative internal discussions may be exempt), they're obligated to provide it — public recruitment processes are generally expected to be reasonably transparent and accountable.

If you don't receive a satisfactory response, or the authority denies the information without adequate justification, you can file a first appeal with a senior officer in the same department, and if still unsatisfied, a second appeal with the Information Commission. Drafting your RTI request clearly and specifically (rather than too broadly) generally gets you a more useful and complete response.