Hello Sourabh,
it it still used by service providers that had done huge investements on it.
ATM has some niches where it is widely used like on xDSL broadband access.
It is wise to get some basis on ATM PVCs, AAL5 and ATM CoS you can skip ATM SVCs, PNNI or LAN emulation just to say.
It it still important also if you think that MPLS is a sort of logical ATM and has its roots in ATM.
Hope to help
Giuseppe