On the other hand, I would say using a local DNS cache can increase both your relay's performance and perhaps offers a slight privacy gain to tor clients, given that a cached DNS response will be served directly to a tor client rather than querying an external resolver for the 2nd time.
Note that, whenever possible, Tor relay operators using a local DNS resolver should enable qname mininisation [1], so that the resolver only sends to the authoritative servers what they need to know to respond. Support for qname minimisation has recently been added in unbound [2] 1.5.7, and is planned in the future Knot resolver [3].
[1]: https://tools.ietf.org/html/draft-ietf-dnsop-qname-minimisation-08 [2]: https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=648 [3]: https://github.com/CZ-NIC/knot-resolver