I've looked through the code and I can't find anything suggesting that MQTT.js supports lookup by SRV records.

There are four SRV combinations listed by IANA for MQTT, {mqtt, secure-mqtt} × {tcp, udp}.

If it doesn't, it should, given a server hostname example.com, first look for _secure-mqtt._tcp.example.com, followed by _secure-mqtt._udp.example.com, and if one of those is found, connect as if a URI had been given with a mqtts scheme. Only if neither of those exist, should it look for the mqtt versions (and only if allowed by an option, which if disabled (by default) would prevent fallback to insecure communication).

Additional context
Given #2002, now might be a good time to implement this.