Well of course, you can pursue Aeronautical as PG degree being UG in Mechanical. Its a good choice though. If you check out the Univerisity requirements, Mechanical graduates are highly encouraged to apply in PG Aeronautics. However, just be a little careful as job in Aeronautics is not wide open, as far as I know. I am a grad of Aero. So I love Aero. But Aero is not that versatile, because its specific just for aircrafts.
My recommendation would be do not go for PG in Aero..keep up with Mechanical or systems.