Yes dear there is no relationship between UG and PG course reservation.Both are different and for PG course you are now a completely fresh candidate.So,you can avail the reservation if you have a valid Caste certificate.Generally the Caste certificate should not be older than 6 months.So, don't worry and enjoy the reservation.
Thanks.

Hello dear student,
Greetings,
Yes of course you can take the advantage of belonging to reserved category while taking admission in pg . Actually it's totally upon student,  if a student belongs to  reserved category but doesn't want to take its advantages then he can simply apply as general candidate but in case he wants to enjoy his reservation he can simply apply as reserved candidates, but yes he must possess caste certificate etc for enjoying his reservation.
Thankyou
Good luck