Is lattice-based cryptography relevant in symmetric cryptography?
No, it is not, assuming you are talking about symmetric ciphers, pseudorandom generators, and hash algorithms.* Symmetric cryptography does not need to use any mathematical hardness problem to be secure, and there is no need to have a security reduction to one. It is technically possible to design a symmetric algorithm out of a core construct that is normally used for asymmetric cryptography, as in the very slow Blum Blum Shub or the backdoored Dual_EC_DRBG pseudorandom generators, but that's rare. Such constructions have no real advantages over typical symmetric ones.

Note that you do not need dedicated post-quantum cryptography to protect your symmetric encryption keys. The quantum Grover's algorithm does not hit symmetric cryptography nearly as hard as Shor's algorithm hits asymmetric cryptography. Even with extremely high-speed and low-latency quantum computers, it would suffice to double your key length from 128 to 256 to remain safe.

* If you consider homomorphic encryption to be symmetric, then the answer might very well be yes.