Could it be possible to reverse engineer an intel sgx application executable to get any meaning text? If yes, how it is secure if a adversary say a system admin could see the details of it?
Yes you can decompile the executable. That isn't what SGX is designed to protect against. Short of encrypting code in a way that only the hardware holds the key to decrypt it you can always reverse engineer an executable.
What SGX gives you is protection against a running program being read or modified by another process / user. Don't forget the executable does not have to be present on persistent media once it has been loaded into memory. You can run an entire system in RAM only. If it were loaded into SGX enclaves it should be protected from the rest of the system.