IIT Mandi M.Tech in Mechanical Engineering with an Energy System specialization has a good reputation. Only Mechanical or Production undergraduates are allowed to apply for this course. The curriculum has fundamental Mechanical engineering courses like Energy sources, Design & Energy systems, Mechanics of Energy systems, Modelling & Simulation, CFD, etc.
The faculties are great. If you are passionate about research, you will get support from them. IIT Mandi gives an option to pursue M.Tech thesis in TU 9 German Universities of Technology. You will need a pointer over 8.5-9 to get the opportunity. Students from this branch get admission in IISc, IIT B, IIT KGP, etc. for Ph.D.
An M.Tech from IIT Mandi is not recommended in the current state. It is always good to aim for the old IITs.
If you are getting an old IIT or a comparable university, it is better to remove IIT Mandi from your prospective list.